Bring 3 cups of water to a boil in a large pot. Add salt and penne pasta. Cook for 10–12 minutes until al dente. Reserve 2–3 tablespoons of pasta water before draining.
Heat butter or olive oil in a saucepan over medium heat. Add minced garlic and sauté for about 30 seconds until fragrant.
Add broccoli, bell pepper, and sweet corn. Sprinkle a little salt and cook for about 2 minutes while stirring. The vegetables should remain slightly crisp.
In another saucepan, melt butter over low heat. Add flour and stir continuously to form a smooth paste. Cook for about 1 minute until the raw flour smell disappears.
Slowly pour in a small amount of milk while whisking constantly to prevent lumps. Gradually add the remaining milk while continuing to whisk.
Cook the sauce for 4–5 minutes while stirring until it thickens. Add heavy cream, grated cheese, oregano, chili flakes, black pepper, and salt. Stir until the cheese melts and the sauce coats the back of a spoon.
Add the sautéed vegetables and cooked penne pasta into the sauce. Toss gently so the sauce coats the pasta evenly.
If the sauce becomes too thick, add a few tablespoons of the reserved pasta water and mix well.
Transfer to a serving plate and optionally sprinkle extra cheese and chili flakes. Serve immediately while warm and creamy.