Preheat the oven to 180C, fan-forced. Cut the unpeeled potatoes into 4mm-thick slices. Thinly slice the onion. Crush or finely chop the garlic. Trim the beans. Finely grate the lemon zest, then cut the lemon into wedges. Finely chop the parsley leaves and stems.
Step 2 — Cook potato and onion
Heat 2 tsp olive oil in a medium deep frypan over medium-high heat. Cook the potato and onion, stirring occasionally, for 6 mins or until the potato is partially cooked.
Step 3 — Make bechamel
Melt 20g butter in a small saucepan over medium heat. Add 1 tbs flour and cook, stirring, for 2 mins or until golden. Gradually add 125ml (½ cup) milk and 250ml (1 cup) water, stirring constantly to prevent lumps forming. Bring to a simmer and cook, stirring, for 1-2 mins until thickened. Season with salt and pepper.
Step 4 — Bake potato bake
Put the potato and onion in a 1L (4 cup) baking dish and pour over the bechamel. Combine the breadcrumbs, cheese and 1 tsp olive oil in a bowl and season with salt and pepper. Scatter the crumb mixture over the potato. Bake for 25-30 mins until golden and tender.
Step 5 — Cook tuna
Once the potato bake has been baking for 15 mins, rub the tuna with 2 tsp olive oil and season with salt. Heat a medium frypan over high heat until hot. Reduce the heat to medium-high and cook the tuna for 2-2½ mins each side for well done, or less until cooked to your liking. Remove from the pan.
Step 6 — Cook beans and serve
Heat 2 tsp olive oil in the same pan over high heat. Cook the garlic and beans, stirring regularly, for 3-4 mins until tender. Put the lemon zest, parsley and capers in a large bowl. Add the tuna and garlic beans and turn to coat. Season with salt and pepper. Divide the tuna and beans and potato bake among plates and serve with the lemon wedges.