Heat some olive oil in a pan and cook the tuna on both sides, add small chili pieces at the end. Steam the green beans, asparagus and broccoli until cooked and leave to cool afterwards. Heat the butter in a pan and cook the mushrooms. Deglaze with a sprinkle of soy sauce and leave to cool, too.
For the sauce cut the onion into very small pieces and press the garlic. Add about a tablespoon of olive oil and two tablespoons of sushi vinegar. Add the mustard and season with salt and pepper.
Mix the vegetables and the sauce and serve with the tuna steak (sprinkle with lemon juice).