Saute the onion, celery and carrots in olive oil until they are lightly browned.
Add the remaining ingredients (except the parsley) to your pot and stir to combine them well.
Add 1 to 2 cups of water, aiming to start with the consistency of chunky tomato soup. Be aware that the lentils will absorb water over time, and the vegetables release water as they break down.
Cooking is done when the lentils are al dente. 30 mins in a sealed pressure cooker, or 45 to 60 minutes on the stovetop on medium heat. If you're cooking on a stove, stir occasionally.
Taste the sauce, and add salt to taste.
Serve on your favourite pasta and garnish with finely chopped parsley.