Step 1: Bring a large pot of salted water to a boil and cook spaghetti according to package directions until al dente. Reserve 1/2 cup of pasta water before draining.
Step 2: While pasta cooks, melt butter with olive oil in a large skillet over medium heat.
Step 3: Add minced garlic to the skillet and cook for 1-2 minutes until fragrant but not browned.
Step 4: Add the drained spaghetti to the skillet along with the reserved pasta water.
Step 5: Toss the noodles in the garlic butter sauce until well coated and the sauce slightly thickens.
Step 6: Remove from heat and stir in chopped parsley, Parmesan cheese, and red pepper flakes if using.
Step 7: Season with salt and black pepper to taste and serve immediately.