Chop onion, celery, carrots, and rosemary. Mince garlic.
Put olive oil, salt, and pepper to your liking and start sautéing the onions, carrots, celery, rosemary, and garlic.
As the rosemary begins to release its aroma and enhance the flavors, you'll feel like a chef! Use a great non-stick pan and wooden spoon to add the meat and tomato paste. You'll have time to season it more without burning your food. Keep it on medium heat while simmering.
This is also a good time to start boiling water for the pasta and heavily salt it. Normally, you would add olive oil, but since the recipe does not call for it and you've never worked with fresh pasta, you skip it to avoid changing the texture.
Add the meat and tomato paste to the sautéed vegetables.
Before cooking the pasta, separate it and add it to the boiling water. Cook it for 3-4 minutes, then strain it over a measuring cup, saving 1 ½ cups of pasta water. Be careful, as it gets very hot. Place the strained pasta on top of the meat in the saucepan, followed by the pasta water and the outer leaves of the Brussels sprouts. Add more salt and pepper to taste as the Brussels sprouts soften before adding the pre-grated Parmesan cheese.