Boil the spaghetti according to the instructions on the package.
Roast the garlic. Cut the upper part of every garlic head (see video), place the garlic on an aluminum foil, drizzle with olive oil and season with salt and pepper, then wrap it together and bake in the oven at 250C for 40 minutes.
In a blender, add the roasted garlic, yogurt, milk, nutritional yeast, flour, salt and pepper. Process until silky smooth.
Pour the sauce into a pan and stir continuously until it thickens. If it gets too thick, add more milk.
Remove from heat and add the drained spaghetti. Mix well and serve with a drizzle of olive oil on top, some chopped herbs and optional, a sprinkle of ground, dry rosemary.