Cajun Salmon Pasta

Name: Cajun Salmon Pasta
Ingredients: 12 ounces spaghetti or fettuccini pasta; 1 cup reserved pasta water*; 8 ounces salmon (about 2-3 fillets, fresh or frozen (thaw it first)); Salt and pepper (for seasoning); 1 tablespoon olive oil; 1 tablespoon butter; 1 small onion (thinly sliced); 2 garlic cloves (minced); 14 ounce can diced tomatoes (drained); 2 cups fresh spinach (chopped); 1 1/2 tablespoons Cajun seasoning; 1/2 cup heavy cream or half & half*; 1/4 cup Parmesan cheese (optional for serving)