Preheat your oven to 200°C (395°F).
Clean the fish thoroughly. Remove the scales with a scaling or butter knife, take out the gills, and rinse the mullet. Clean the belly cavity, removing the black lining and bloody kidney line. Rinse again under cold water to ensure it's completely clean.
Prepare a baking sheet with olive oil. Peel and slice the potatoes into 0.5 cm (0.2 inch) thick rounds and place them on the sheet. Slice the onions, peppers, tomatoes, garlic, and lemon.
Place the cleaned fish on the potatoes, seasoning it generously with salt on both sides and inside the belly cavity.
Add the onions, garlic cloves, and sliced peppers to the tray. Sprinkle with salt and mix well with the potatoes to evenly distribute them around the fish.
Stuff the belly cavity with a few garlic cloves, lemon slices, and fresh parsley. Optionally, place a lemon wedge in each gill opening.
Add sliced tomatoes to the tray and season everything with black pepper and dried rosemary. Place a few lemon slices on top of the fish and drizzle with olive oil for a crunchy finish.
Bake for about 40 - 45 minutes at 200°C (395°F).
After 25 minutes, carefully flip the fish over and stir the vegetables for even cooking.
Add tomatoes and lemons on top of the mullet for added juiciness. Drizzle with olive oil if you prefer a crispy skin.
While baking, make the sauce. Finely chop the garlic and sauté in hot olive oil for about a minute. Add fresh chopped parsley, a pinch of salt, and a squeeze of lemon juice.
When done, transfer everything to a plate, drizzle with the garlic sauce, and garnish with fresh basil leaves. Serve immediately and enjoy!