Sofrito. Heat most of the olive oil in a paella pan over medium heat. Add the onion, green pepper, and garlic; sauté a few minutes until soft.
Add the grated tomatoes, mushrooms, salt, paella seasoning, and pepper. Cook down the tomato juice thoroughly — reduce until thick and dark with no watery juice left, about 8-10 minutes. Then add the white wine and let it reduce fully.
Pearl the rice (second pan). Meanwhile, heat the remaining olive oil in a separate pan. Add the dry rice and toast, stirring, about 2 minutes, until the grains look pearly/translucent at the edges and smell a little nutty. Do not brown it.
Combine. Add the pearled rice to the paella pan. Pour in the hot vegetable stock. Distribute the rice evenly in one thin layer. Taste for salt — this is the last chance to adjust. Do not stir again from this point.
Cook UNCOVERED over medium heat about 18-20 minutes, until the stock is absorbed and the rice is just tender (pearl/round rice cooks in about 18 minutes).
Socarrat. Raise the heat to high for 60-90 seconds until you hear gentle popping or crackling. If it smells like burning, pull it off immediately.
Rest off the heat a few minutes (loosely covered with a towel is fine, no lid while cooking), then serve from the pan. Lemon wedges optional.
