Preheat oven to 375°. In a large pot of boiling salted water cook pasta until almost al dente, about half the time called for on box. Drain.
In a large skillet over medium heat, melt butter. Add garlic and cook until fragrant, 1 minute. Add flour and stir until golden and fragrant, about 1 minute more. Slowly add milk and whisk until no lumps remain.
Add ½ cup mozzarella, Parmesan, and lemon juice and simmer for 5 minutes. Add spinach and artichokes, stirring until well coated. Season with salt, pepper, and a pinch of red pepper flakes.
Add cooked tortellini and toss to combine. Pour pasta into a large baking dish and top with remaining ¾ cup mozzarella.
Bake until cheese is melty and sauce is bubbly, 20 minutes.
