Step 1 Place cauliflower into a large pot of salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 10 minutes. Remove cauliflower with a strainer, reserving the cooking water in the pot.
Step 2 Melt butter in a large skillet over medium-high heat. Add onion and cook until transparent, about 5 minutes. Add cauliflower florets and red pepper flakes; saute until browned, 7 to 10 minutes. Add heavy cream; cook until reduced by half, 10 to 15 minutes.
Step 3 Meanwhile, bring the cauliflower cooking water back to a boil. Add farfalle and cook, stirring occasionally, until tender yet firm to the bite, about 12 minutes. Drain.
Step 4 Add tomatoes to the cauliflower sauce; cook until heated through, about 1 minute. Add 1 cup Parmesan cheese to the cauliflower sauce; cook and stir until melted, about 1 minute more.
Step 5 Combine pasta and sauce; toss until well coated. Sprinkle with additional Parmesan cheese to serve.