Preheat the oven to 400 degrees F (200 degrees C). Place baguette slices on a baking sheet.

Heat a large skillet over medium-high heat. Cook and stir sausage until browned and crumbly, 5 to 7 minutes. Transfer to a plate.

Cook and stir onion in the same skillet until softened, 3 to 5 minutes. Add eggs and sausage; stir until eggs are set, about 5 minutes. Season with salt and pepper.

Divide egg mixture among baguette slices and sprinkle Cheddar cheese on top.

Bake in the preheated oven until cheese is melted, 8 to 10 minutes.