Instructions

1. Arrange a rack in the upper third of the oven and heat the oven to broil on high. Meanwhile, prepare the sandwich filling.
2. Slice the ciabatta(s) in half lengthwise. Thinly slice 1 small lemon and remove any seeds. Thinly slice 2 medium shallots and 2 large garlic cloves. Drain and thinly slice 1 jar roasted red peppers. Pick the leaves off fresh oregano sprigs until you have 2 tablespoons.
3. Heat 1 tablespoon of the olive oil in a large nonstick skillet over medium-high heat until shimmering. Add the lemon slices in a single layer and sear until dark golden brown, 2 minutes per side. Transfer to a cutting board and finely chop.
4. Reduce the heat to medium and add the remaining 1 tablespoon olive oil. Add the shallots and cook until softened, 4 minutes. Add the garlic, oregano, and caramelized lemon, and cook until fragrant, about 1 minute.
5. Add 10 ounces baby spinach and 1/4 cup water, and scrape up any browned bits from the bottom of the pan. Toss until the spinach is almost wilted, about 2 minutes. Crumble in 8 ounces feta cheese with your hands and toss until the feta is warmed through and starting to melt, about 2 minutes. Taste and season with kosher salt and black pepper as needed. Transfer the mixture to a bowl (reserve the pan).
6. Place the ciabatta(s) cut-side up on a baking sheet. Broil until golden brown and toasted, 1 to 2 minutes. Divide 4 ounces of the sliced mozzarella cheese onto the bottom halve(s) of the bread. Divide and spread the spinach mixture evenly over the mozzarella. Top with the red peppers and remaining 4 ounces mozzarella. Close the sandwiches with the top halve(s) of the bread.
7. Wipe out the reserved skillet. Cut the sandwich in half if needed to fit into the skillet. Melt 2 tablespoons unsalted butter in the skillet over medium-low heat. Add the sandwich(es). Cover (it’s ok if the lid doesn’t fit perfectly, as long as it is partially covering the sandwich) and cook until the bread is golden brown and the cheese is melted, occasionally gently pressing down on the sandwich(es) with a rubber spatula, about 5 minutes per side.
8. Transfer to a cutting board. Cut the loaf into 4 sandwiches. Serve immediately.