Cut the salmon into bite-sized pieces.
Wash the spinach thoroughly and remove excess water.
Finely chop the onion and garlic.
Heat olive oil in a pan and sauté the onion and garlic.
Add the spinach and salmon, and stir-fry lightly.
Melt butter, then add flour to make a roux.
Pour in the milk and simmer gently until smooth.
Add the sautéed ingredients and heavy cream, and mix well.
Season with salt and pepper.
Transfer to a gratin dish and top with cheese.
Bake in a 200°C oven for 15 minutes.