These lentils are even better the next day, so we’ll often make a pot with the intention of not serving them for a day or two. When reheating, you may want to add a little extra water — the lentils can settle in and soak up a bit of the liquid as they sit.
Put a kettle or a pot (containing about 4 cups water) on the stove and bring to a boil.
Meanwhile, heat the oil in a medium saucepan over medium heat until shimmering. Add onion and cook until soft and translucent, about 5 minutes. Stir in the garlic and cook for an additional minute. Fold in the carrots and celery.
Add the lentils to the saucepan and stir to coat with the onion mixture. Pour the boiling water into the pan and stop when the level is about ½-inch over the top of the lentils (should be roughly 3 1/2 cups of water). Bring back to a good boil, reduce the heat to low, cover the pot and cook the lentils for about 20 minutes. Stir in the mustard, vinegar, salt and pepper and cook an additional 10-15 minutes – or until lentils are tender but still slightly chewy.
Remove from the heat and let sit for ten minutes. Taste and season with additional salt and pepper.
Serve warm – with a lightly-fried egg and a sprinkling of parsley on top if you’d like. Leftovers can be covered and refrigerated for an additional 4-5 days.