Preheat the oven to 350 degrees . In a food processor, puree 2 cups spinach with the pesto; season with salt and pepper.
In a medium bowl, whisk together the eggs, half-and-half, pecorino-romano and scallions. Season with salt and pepper.
In a nonstick skillet, cook the sausage over medium-high heat, breaking it up, until cooked, about 7 minutes. Using a slotted spoon, transfer to a bowl. Reduce heat to medium and cook the remaining spinach until wilted, about 1 minute; chop.
Using half of the ingredients: Arrange bread in a foil-lined 7-by-11-inch baking dish, spread on pesto, then ricotta; pour in egg mixture and top with spinach, sausage and fontina. Repeat. Let stand for 10 minutes, pressing to soak.
Bake the strata until puffed and browned, about 45 minutes. Let stand for 10 minutes before serving.