Preheat the oven to 375oF (190oC) and line a baking sheet with parchment paper.
In a bowl, mix together the spinach, mozzarella cheese, roasted red pepper, green onion, garlic, lemon juice, salt, and pepper.
Roll out the puff pastry into a square, about 1/4-inch (1/2 cm) thick.
Spread the spinach mixture over the puff pastry. Roll it up with the seam side down, then cut into 2-inch (5 cm) pieces. Place the rolls on the prepared baking sheet.
Brush the tops with egg yolk and sprinkle with sesame seeds. Bake for 22 to 25 minutes, or until golden brown. Serve warm and enjoy!