In a large pot of lightly salted boiling water, cook the penne according to package directions.
Before draining the pasta, dip a glass measuring cup into the cooking water and measure out 1 cup of the water; set aside.
Drain the pasta and set aside.
Meanwhile, in a large skillet over medium-high heat, melt the butter or margarine.
Add the olive oil, onion and garlic.
Cook, stirring occasionally, until the onion begins to brown, about 7 minutes.
Add the crushed red pepper and the cauliflower; cover and cook, stirring occasionally, for 5 minutes.
Add ¾ cup of the reserved cooking water and continue cooking until the cauliflower is tender, about 5 to 7 minutes.
Add more cooking water if needed.
Add the penne, Parmesan, parsley, lemon juice, salt and pepper, stirring well to mix.
Cook until the pasta is heated through and the cheese is melted, about 2 minutes.