Wash the trout fillets and dry with paper towels. Place them in a dish in a single layer. Pour on the lemon juice and cover the dish with plastic wrap. Refrigerate for 2 hours.
When you are ready to cook, prepare all of the vegetables. Heat the oil at medium temperature in a skillet (large enough to hold the trout fillets). Add the onion and garlic and sauté gently until the onion is tr