Cook the pasta according to package instructions until al dente. Reserve ½ cup pasta water, then drain.
In a large skillet, heat olive oil and butter over medium heat. Add garlic and onion; sauté until fragrant.
Stir in red pepper flakes and cook for another minute.
Add white wine and simmer until reduced by half.
Pour in broth and cherry tomatoes; cook for 3–4 minutes.
Add shrimp and scallops; cook until opaque and tender.
Season with salt, pepper, lemon juice, and parsley.
Toss pasta into the sauce, adding reserved pasta water if needed for consistency.
Serve immediately topped with Parmesan, if desired.