Chop your basil and rosemary, and shred the cheese ahead of time. Once you start making the sauce, things will start to move quickly.
Fill a medium pot with water about half way and bring it to a boil.
Once the water is boiling, lower the heat a bit and add your pasta. Make sure you add a generous amount of salt to your pasta water for flavor, and to prevent the water from boiling over.
Once the pasta is al dente, set aside about 1/4 cup of pasta water. You're going to add that to the sauce later. Drain the rest, and set the pasta aside with a tea towel over it. This will keep the pasta warm while you make the sauce.
Dice up a quarter of an onion and toss into the same large pot you cooked the pasta in. Add butter (1 tbsp) to the pot and cook the onions until they're fragrant.
When the onions are translucent, add in the cream (1 cup), herbs (6 basil leaves, 2 rosemary springs), minced garlic (2 cloves), and salt and pepper.
Lower the heat a bit. When the cream starts to simmer, stir for about 5 minutes.
Use the emersion blender to blend the onions into the sauce. This helps make everything smooth.
Stir in the gruyere and cheddar cheese to the pot. The sauce will start to become very thick as the cheese melts. Stir in a little bit of milk at a time until you have your desired consistency.
After the cheese has melted add in the pasta water. This will help the sauce stick to the pasta.
Toss the pasta into the creamy sauce until all of it is coated evenly.
And there you have it! Creamy, delicious, herby mac n cheese.