How to Make It
Light a grill or preheat the broiler.
In a saucepan, heat 2 tablespoons of the olive oil. Add the onion and cook over low heat until softened but not browned, about 5 minutes. Add the yellow peppers, potato, water and a large pinch of salt. Cover and cook until the peppers and potatoes are tender, about 12 minutes. Transfer the vegetables and their liquid to a food processor and process until roughly pureed. Scrape the sauce into a bowl, add the lemon juice and season with salt and pepper.
Brush the tuna steaks with olive oil and season with salt and pepper. Grill or broil for 2 minutes on each side, or until lightly charred on the outside and medium rare in the center. Spoon the sauce onto large plates and set the tuna steaks on top. Garnish with sage leaves and serve.