Preheat oven to 350°F and lightly grease a 9x13-inch baking dish.
Cook spaghetti in salted water until al dente, then drain.
In a large skillet, melt butter over medium heat. Sauté onion until soft, then add garlic and cook for 1 minute.
Stir in flour and cook for 1 minute. Slowly whisk in chicken broth, then milk and cream. Cook until thickened.
Season with salt, pepper, and paprika. Stir in chicken, peas, sour cream, and 1 cup mozzarella.
Add cooked spaghetti and toss to combine. Spread mixture into the prepared dish.
Top with remaining mozzarella, Parmesan, and a mixture of panko and olive oil.
Bake for 25-30 minutes, until golden and bubbly. Cool slightly before serving.