Melt butter in a large saucepan over medium heat.

Add chopped onions and carrots to the saucepan.

Cook the onions and carrots for 5 minutes, until softened.

Pour beef stock into the saucepan.

Stir in tomato paste.

Mix well to combine.

Season with salt and pepper to taste.

Add lentils, minced garlic, and chopped parsley to the soup.

Bring the soup to a boil.

Reduce heat to low.

Cover the saucepan partially.

Simmer the soup for 1 1/2 hours, or until lentils are tender.