Heat a heavy-bottomed pan over medium-high heat.
Add the sausage pieces and brown until cooked through, about 8 minutes.
Add the onions and cook until lightly browned. Remove the pan from the heat and stir in the spinach.
In a large mixing bowl, whisk together the eggs, half and half, Fontina, and pesto until thoroughly combined.
To assemble: Grease a casserole dish with pan release and place the bread in the bottom. Top with the sausage mixture, then top with the egg mixture.
Refrigerate for at least 30 minutes or up to 24 hours.
Preheat the oven to 350ºF.
Bake the casserole uncovered until lightly browned and set, about 30 minutes.
Remove from the oven and serve immediately.