Preheat oven to 450°F. Heat 2 tablespoons olive oil in a medium sauté pan and add onion, garlic, and peppers. Sauté for 3 minutes, stirring frequently. Add stock or wine, herbs, and saffron (optional), and season to taste with salt and pepper. Bring to a boil, then spoon pepper mixture into a baking dish large enough to accommodate fish fillets in one layer.