Step 1: In a non-stick skillet, heat over medium heat and add chopped bacon. Cook for about 8–10 minutes, stirring occasionally, until browned and crispy. Transfer bacon to a plate lined with paper towels to drain excess grease.

Step 2: In the same skillet with reserved bacon fat, add diced yellow onion and sauté for 3 minutes until translucent. Incorporate grated garlic and baby spinach, cooking for an additional 2–3 minutes until spinach wilts. Remove from heat to cool slightly.

Step 3: In a large mixing bowl, whisk together eggs, egg whites, Dijon mustard, fresh thyme leaves, ground nutmeg, milk, and plain Greek yogurt until smooth. Season with kosher salt and fresh ground black pepper to taste.

Step 4: Grease a 3-quart baking dish. Layer one-third of the bread cubes at the bottom, followed by half of the spinach mixture, half of the crispy bacon, and one-third of the Gruyere and smoked mozzarella. Repeat, finishing with cheese on top.

Step 5: Pour whisked egg mixture evenly over the layers. Cover tightly and refrigerate for at least 8 hours or overnight.

Step 6: Let the strata sit at room temperature for 30 minutes before preheating the oven to 350°F. Bake uncovered for 50–65 minutes until golden. Cover loosely with aluminum foil if browning too quickly.

Step 7: Allow the strata to stand for 5 minutes after baking, then cut into squares and serve warm.