Preheat oven to 400°F (200°C). Line a large rimmed baking sheet with parchment paper.

In a large skillet, melt butter over medium heat. Add garlic, and cook until fragrant, about 30 seconds. Add spinach in batches, and cook, stirring frequently, until almost wilted, about 2 minutes. Stir in cream, flour, and salt, and cook until slightly thickened, about 2 minutes. Remove from heat and let cool completely.

On a lightly floured surface, roll Cheesy Piecrust into a 12-inch circle (about ¼ inch thick). Trim ¼ inch off edges, if desired. Transfer to prepared pan. Using a fork, pierce dough in ½-inch intervals. Spoon spinach mixture onto center of dough, and spread into an even layer, leaving a 1-inch border around edges. Fold edges over spinach mixture in 2-inch sections, overlapping, and pinch to seal any gaps. Using a pastry brush, brush edges with egg.

Bake until dry and slightly golden on edges, 12 to 15 minutes. Rotate pan, and reduce oven temperature to 350°F (180°C). Bake until crust is golden, 5 to 8 minutes more. Let cool on pan for 5 minutes. Garnish with red pepper and cheese, if desired. Serve immediately.