Start a pot of salted water for the linguine over high heat. When it is boiling, cook as per package directions until al dente.
Meanwhile, in a large pot, melt the butter over medium heat.
Add the diced onions and cook until translucent, approximately 3-5 minutes.
Add the flour to the butter and onions. Whisk to combine and cook until it is a light brown color, approximately 2-3 minutes.
Remove the roasted garlic from the skins into a small bowl. Mash the garlic with a fork until it is a paste. Some chunks are okay.
Whisk in the roasted garlic, walnuts, spinach, and sun-dried tomatoes to combine. Add the milk, garlic powder, salt, and pepper and whisk together. Simmer, stirring occasionally, until the sauce just starts to thicken, approximately 5-8 minutes.
Turn heat down to medium-low and add the Parmesan cheese. Simmer until the sauce starts to thicken, approximately 5-8 minutes. Season to taste with salt and pepper, cheese, or tomatoes.
When your sauce is thickened and seasoned, add the pasta to the sauce pot. If the sauce is too thick, add a little of the pasta water or a little more milk.
Notes
Learn how to make roasted garlic here!The roasted garlic is the star here, but if you are pressed for time and do not have roasted garlic, you can use three minced cloves of fresh garlic instead (but you will be missing out on some flavor!).If trying to keep it truly vegetarian, make sure you are using vegetarian cheese.As with all recipes, season to your taste preference with the cheese, salt, pepper, and garlic powder. Everyone likes things a little different, and you can make it perfect for yourself when you taste along the way!I think the sauce tastes a lot better the next day after the flavors have time to meld together. So if you can plan in advance, you can make the sauce the morning of or even the night before. Then when ready cook the pasta as per package directions, drain, add the sauce and stir to coat until heated through. Yum!*Note: This recipe was created using fresh Parmesan cheese. The fresh cheese thickens the sauce and melts in better (while also tasting better!).