043 / 75 · Chicken, Turkey and Fish Suppers
Tuna and Green Bean Potato Gratin
One 8×12 in (20×30 cm) dish, at least 3 qt/2.8 L;4 main-dish portions · Active 35 min · Total 1 hr 30 min
Print recipe · PDFPotato rounds and green beans are par-cooked together before joining tuna in a light sauce. A small amount of caper and lemon zest sharpens the filling, while Gruyère and fresh crumbs give the gratin a golden top. The fish begins canned, so the oven stage is kept short instead of waiting for raw potatoes to soften around it.
Let the potatoes start first
Slice potatoes evenly and bring them to a gentle simmer. Green beans enter later because their shorter pieces need less time. Check both before draining: the potatoes should bend without falling apart and the beans should be nearly tender. Drain carefully rather than pressing the slices through the colander. The shared saucepan is useful, but it does not mean both vegetables enter raw at the same moment. Having them prepared lets the milk sauce do its own job of binding instead of carrying extra cooking water into the bake.
Keep the tuna in the middle of the process
Drain plain tuna in water, inspect it and leave broad flakes. Make the sauce smooth first, take it off the heat and stir in zest, parsley and chopped capers. Then fold the fish through gently. Repeated stirring breaks it into fine fibers that disappear between potato slices. Capers and cheese supply seasoning, so the base has no added salt. Oil-packed tuna or a flavored tuna salad would have a different starting fat and dressing balance; plain fish gives this recipe a clear quantity.
The top is a light finish
Fresh coarse crumbs and grated Gruyère are scattered rather than pressed into a solid lid. A little oil coats them, and gaps let steam escape. The pan needs only enough oven time to color the surface and reach the casserole endpoint throughout. If the top gets ahead, shield loosely instead of allowing it to burn while the center catches up. Rest for five minutes, then spoon through all the components so no plate receives only the attractive potato surface.
Ingredients
- 1 lb 10 oz waxy potatoes, peeled and sliced 1/4 in/6 mm thick (750 g prepared)
- 8 cups water (1.9 L), for vegetables
- 10 oz fresh green beans, trimmed and cut in 1-in/2.5-cm pieces (300 g prepared)
- 2 tablespoons butter (30 g)
- 1 small onion, finely chopped (120 g)
- 3 tablespoons all-purpose flour (25 g)
- 1 1/4 cups milk (300 ml)
- 3/4 cup low-sodium vegetable stock (180 ml)
- 1 tablespoon capers, drained and chopped (10 g)
- 1 teaspoon finely grated lemon zest (2 g)
- 2 tablespoons chopped fresh parsley (8 g)
- 1/4 teaspoon black pepper
- 12 oz plain canned tuna in water, drained and inspected (340 g drained)
- 2 oz Gruyère, grated (60 g)
- 2/3 cup coarse fresh breadcrumbs (40 g)
- 1 tablespoon olive oil (15 ml)
Set out
Oven, 8×12 in (20×30 cm) baking dish, at least 3 qt/2.8 L, Large saucepan, Colander, Medium saucepan, Whisk, Small bowl, Rimmed baking sheet, Suitable foil if needed, Food thermometer
Allergens
Fish: tuna, Milk, Wheat; check stock and bread labels. Check labels and substitutions.
Make it
- Heat oven to 400°F (205°C). Put the dish on a rimmed sheet. Bring potatoes and their water to a gentle boil. Simmer 4 minutes, add green beans and simmer 4–5 minutes more until potatoes bend and beans are nearly tender. Drain carefully.
- Meanwhile melt butter in the medium pan over medium-low heat. Cook onion 7–9 minutes until soft. Stir in flour 1 minute, gradually whisk in milk and stock and simmer 3–4 minutes until sauce coats a spoon. Take off the heat and stir in capers, zest, parsley and pepper. Fold tuna in gently.
- Arrange about half the potatoes and beans across the dish, add the tuna sauce, then distribute remaining vegetables over and through it without compressing. Mix Gruyère, crumbs and oil in the small bowl and scatter over the surface.
- Bake 25–30 minutes until golden and the center and several other spots reach 165°F (74°C). Shield crumbs loosely if needed while the middle finishes.
- Rest 5 minutes and divide into four portions.
A shared vegetable pan still has an order
Set a timer for the potato head start before adding beans. Four minutes is a useful instruction to follow, not a reason to put every ingredient into cold water together.
A useful detail
The 90-minute total includes preparing tuna and vegetables, overlapping their saucepan stages, assembly, the full bake and rest. The potatoes are partially cooked before assembly; raw slices would extend the fish’s oven time.
Another way
- Replace tuna with 340 g drained inspected plain canned salmon. Keep the same vegetables, sauce and final 165°F (74°C) endpoint and update the fish label.
Keep it well
Refrigerate shallow portions within 2 hours, or 1 hour above 90°F (32°C), at 40°F (4°C) or below. Use within 2 days or an earlier opened-fish or dairy deadline. Reheat to 165°F (74°C). Refrigeration is preferred to freezing this potato-and-milk sauce; the crumb surface softens after storage.