Preheat the oven to 200°C (390°F) using conventional oven settings (top and bottom heat). Halve the shallot, peel it, and slice it into very thin rings. Wash the spinach and spin it dry (or drain it thoroughly).

Mix yogurt, oil, grated garlic (optional), and spices in a bowl and season generously with salt and pepper. Rinse the fish under cold water, pat dry with paper towels, and cut into slices about 2 to 3 centimeters thick.

Add the fish to the marinade and carefully turn it over in the marinade using a dough scraper or similar tool, ensuring that all pieces of fish are completely coated.

Place spinach and onions on a baking sheet, mix with a little extra olive oil (about 1 teaspoon) and spread evenly on the baking sheet.

Distribute the fish pieces evenly over the spinach and bake the tray in the oven at 200°C for 10 minutes, or until the fish is cooked through but the spinach is not burnt. (Your fish is done when it has an internal temperature of 55 to 60°C or is no longer translucent in the center).

Remove the baking tray from the oven and serve the fish fillets and vegetables with rice or a side dish of your choice.