Preheat the oven to 400°F. Lightly grease 2 (4-inch) ramekins with the butter.

In a bowl, add the almond flour, cheddar cheese, Parmesan cheese, eggs, olive oil, onion powder, and black pepper. Divide the mixture between the prepared ramekins. Bake for 15 minutes, or until firm and cooked through. Remove the bread from the ramekins and cut it into halves lengthwise.

In a skillet, melt the butter over medium heat. Add the bread halves to the skillet and toast on both sides, for about 2 minutes.

Add the cheddar slices on 2 pieces of toasted bread. Cover with the remaining pieces of bread.. Toast the sandwich for 2 to 3 more minutes, flipping, until both sides are light brown and the cheese is melted. Serve immediately.