1 medium sweet potato (2 cups cubed)
1/2 medium sweet onion, diced
1 tablespoon olive oil
1/2 teaspoon sea salt, divided
1/2 teaspoon pepper, divided
2 cups spinach, shredded
8 ounces stale whole wheat sourdough bread (7 to 8 thick slices)
4 ounces shredded fontina, divided
1 tablespoon minced fresh rosemary
1 tablespoon minced chives
5 large eggs
1 1/2 cup whole milk
Prep this strata the night before you plan to serve it. Preheat oven to 400˚F. Peel and cut the sweet potato into 1/4-inch cubes. Toss them with the onion, olive oil, 1/4 teaspoon salt, and 1/4 teaspoon pepper. Spread in a single layer on a baking sheet or roasting pan, and roast until the sweet potatoes are tender, 18 to 20 minutes.
Place the spinach in a large bowl and scoop hot sweet potatoes and onions top. Stir, letting spinach wilt just slightly from the heat of the roasted vegetables. Cube the bread and add to the bowl, along with 2 ounces of cheese, the rosemary and the chives. Toss the mixture until well combined.
Lightly grease a 9x9-inch (or 2 1/2 quart) baking dish. Scoop the bread mixture into the pan, evenly distributing the spinach and sweet potatoes. In a separate bowl, whisk together the eggs, milk, remaining 1/4 teaspoon pepper, and 1/4 teaspoon salt. Pour over the bread mixture, pressing down on the bread to submerge it completely in the egg mixture. Cover with foil and refrigerate overnight.
In the morning, remove strata from refrigerator and preheat the oven to 350˚F. Bake the strata for 45 minutes. Remove the foil, sprinkle with remaining 2 ounces of cheese, and continue to bake uncovered for 20 to 30 minutes until the strata has puffed and the cheese is browning. Serve with an extra sprinkle of chives.