In a large pot, bring salted water to a boil. Once boiling, add the linguine and cook according to package instructions until al dente. Reserve 1 cup of pasta water and drain the rest. Set aside.
While the pasta is cooking, heat the olive oil in a large skillet over medium heat. Add the ground beef and sausage. Cook until browned and fully cooked, about 7-10 minutes. Break it apart with a wooden spoon as it cooks.
Add the minced garlic to the meat mixture and sauté for about 1-2 minutes, until fragrant.
Stir in the fresh spinach and cook until just wilted, about 2-3 minutes.
Lower the heat and add the butter to the skillet. Once melted, stir in the red pepper flakes (if using), and stir until well combined.
Add the drained linguine to the skillet and toss to coat with the garlic butter sauce. If the mixture seems dry, add a bit of the reserved pasta water to achieve desired consistency.
Sprinkle in the grated Parmesan cheese and toss again until melted and incorporated. Season with salt and black pepper to taste.
Divide the pasta among serving plates and garnish with additional Parmesan cheese if desired.