Preheat the oven to 375°F (190°C). Heat 1 tablespoon olive oil in a large skillet over medium heat. Add the fresh spinach in batches and sauté until wilted, about 2–3 minutes per batch. Transfer wilted spinach to a fine-mesh sieve or colander, press or squeeze to remove excess moisture, then roughly chop and set aside.

Prepare artichokes

Drain and roughly chop the artichoke hearts into bite-sized pieces. Pat dry to remove excess liquid.

Mix gratin

In a mixing bowl, combine the softened cream cheese, sour cream, mayonnaise, grated Parmesan, shredded mozzarella (reserve a tablespoon for topping if you like extra browning), minced garlic (2 tbs), lemon juice, kosher salt, black pepper, and red pepper flakes. Stir until smooth and fully combined. Fold in the chopped sautéed spinach and chopped artichoke hearts until evenly distributed.

Bake

Transfer the mixture to a shallow ovenproof baking dish (about 9x6 inches or similar). Smooth the top, sprinkle the reserved mozzarella evenly over the surface. Bake in the preheated oven for 18–22 minutes, or until bubbly and lightly golden on top. If you want more color, place under the broiler for 1–2 minutes, watching closely to avoid burning. Remove from oven and let rest 5 minutes.

Toasted Baguette

Slice the baguette on a diagonal into 1/2-inch thick slices to yield about 18–20 slices. Brush both sides lightly with 2 tablespoons olive oil. Arrange slices on a baking sheet in a single layer. Toast in the oven at 375°F (190°C) for 8–10 minutes, flipping once halfway, until edges are crisp and golden. Alternatively, toast under a broiler for 1–2 minutes per side. As soon as the slices come out of the oven, gently rub one side of each slice with the halved garlic clove for a subtle garlic flavor. Keep warm.

Roasted Red Pepper Garnish

If using jarred roasted red peppers, drain and pat dry with paper towels, then slice into thin strips. If using fresh roasted peppers, roast over an open flame or under a broiler until skins char, steam in a covered bowl for 10 minutes, peel, remove seeds, and slice into strips.

Toss the red pepper strips with a pinch of sea salt and 1 tablespoon extra-virgin olive oil. Tear basil leaves if using and set aside for garnish.

Assembly

Spoon a generous tablespoon (or more) of the warm spinach & artichoke gratin onto each toasted baguette slice. Top each crostini with a strip of roasted red pepper and a small torn basil leaf if desired. Finish with a light drizzle of extra-virgin olive oil and an optional sprinkle of additional grated Parmesan or cracked black pepper.

Arrange crostini on a serving platter and serve warm. Leftover gratin can be refrigerated up to 3 days; rewarm in a 350°F (175°C) oven until heated through before assembling.