Vegetarian ‘Carbonara’ With Spinach

INGREDIENTS

3 tbsp

extra-virgin olive oil

1 cup

finely chopped onion

5 cloves

garlic, thinly sliced

1 lb

spaghetti

Salt and pepper

2 tbsp

unsalted butter

1 lb

fresh baby spinach

Red-pepper flakes, to taste (optional)

3/4 cup

grated Parmesan (2 1/2 ounces)

1 tbsp

fresh lemon juice

1/2 cup

grated smoked Provolone or Gouda (1 1/2 ounces)