Preheat oven to 400°; thinly slice onion, wipe mushrooms clean + thinly slice

Place 2 large pieces of foil on a sheet pan + oil the foil; add 2 fillets onto 1 side of 1 foil + spread each with 2 tbsp sauce, 4 onion slices, 2 tbsp oil + salt/pep; into 2nd foil, add beans, mushrooms + remaining onions; drizzle with 4 tbsp sauce, 2 tsp oil + salt/pep; fold empty sides of foil over ingredients; working from one end, tightly fold edges of foil creating a sealed half-moon shape

Bake veg 20-25 til tender; bake fish for 12-15 min until tuna is firm + cooked through

Open foil packs by cutting a slit through center (watch out for steam!) + divide to plates