Heat a large Dutch oven over medium heat.
Add the olive oil and butter and cook until the butter has melted.
Next, add the carrots, celery, and leeks.
Cook for 5 to 7 minutes or until the vegetables are tender. Stir occasionally.
Stir in the garlic and thyme and cook for an additional minute.
Add the chicken stock and scrape the bottom of the pan to release any browned bits.
Add the lentils, bay leaf, and Dijon mustard.
Bring the soup to a boil and then reduce the heat to medium-low.
Simmer for 30 to 35 minutes, or until the lentils are tender.
Remove the bay leaf and season with salt and pepper to taste.
Enjoy warm. Store any leftover soup in an airtight container in the fridge for up to three days.
