Step 1In a large, heavy, ovenproof casserole, make a sofregit by cooking the onions and garlic in the olive oil very slowly over low heat until tender and golden but not browned, about 40 minutes to an hour.
Step 2Cut the tomatoes in half and grate them by pressing the fleshy side against the medium holes of a box grater. Discard the skins. After the onions are tender, add the grated tomato and saffron and cook slowly for 10 minutes. Heat the oven to 375 degrees.
Step 3Add the rice and stir to coat the grains in the sofregit. Add the white wine, stock, favas, peas, asparagus and salt.
Step 4Bring the rice mixture to a simmer over medium heat. Remove from the stove top and bake, uncovered, until stock is absorbed and rice is cooked, about 20 minutes.
Step 5Remove from the oven and cool for 5 to 10 minutes. Decorate with the piquillo pepper slices and serve with lemon wedges.