Heat olive oil in the pot or casserole and add onions, carrots, celery and garlic, stirring over low heat until vegetables are soft and translucent, about 6 minutes.  Add ground beef, salt and pepper and stir well with a wooden spoon to break up the meat, cooking until it's lightly browned, another 3 to 5 minutes. Add milk, cover the pot and cook over very low heat, stirring occasionally until the meat absorbs most of the milk and gets tender, about 40 minutes. Add tomato paste dissolved in water or wine and continue cooking the sauce over low heat for another 30 to 40 minutes.
Meanwhile, you’ll want to cook the spaghetti or other type of pasta in boiling salted water until it is just done according to the directions. Drain the pasta and mix with meat sauce, topped with a sprinkling of grated Parmigiano-Reggiano cheese. Mangia bene!