Place a large pot with abundant water over high heat. When it starts to boil, add a generous tablespoon of coarse salt and add the pasta. Cook until it's al dente, following the package directions.
Have ready a buttered medium baking dish or several individual-size gratin dishes or ramekins that can go into the oven. Mine is 7x9 inches (18x23cm) and oval.
While the pasta cooks, preheat the oven to 350ºF (180°C).
Mix the sour cream, milk, ½ cup parmesan, ½ cup mozzarella, blue cheese and onions in a medium bowl.
Drain the cooked pasta and add to the prepared dish.
Add the cooked macaroni, season with salt, pepper, thyme, sriracha and mix well.
Top with the rest of the parmesan and mozzarella cheeses, dividing evenly.
Bake for 15-20 minutes, until it bubbles and the cheese melts. Broil for the last 5 minutes to get a deeper golden color, being careful not to burn it.
Serve immediately. Store leftovers in an airtight container.