Heat the olive oil in a large skillet or Dutch oven over medium heat until shimmering and fragrant.
Add the chopped onion and cook, stirring occasionally, until it turns golden and caramelized, about 8-10 minutes. The onions should be soft and sweet-smelling.
Add the minced garlic and cook for 1 minute, stirring constantly until fragrant—avoid burning for a mellow, sweet aroma.
Stir in the grated or finely diced carrots and cook for another 3-4 minutes, letting them soften slightly and release their natural sweetness.
Pour in the crushed tomatoes and stir well, scraping up any browned bits from the bottom of the pan. Bring the mixture to a gentle simmer and cook for 5 minutes, allowing the flavors to meld.
Add the rinsed lentils, vegetable broth, bay leaf, and thyme sprig. Stir everything together and bring to a gentle boil.
Reduce the heat to low and let the ragu simmer uncovered for 25-30 minutes, stirring occasionally. The lentils should be tender, and the sauce will thicken and become glossy.
Remove the bay leaf and thyme sprig. Taste the sauce and season with salt and pepper as needed. Let it sit off heat for 5 minutes to deepen the flavors.
Serve the rich, thick lentil ragu hot over pasta, polenta, or crusty bread. Garnish with fresh herbs if desired and enjoy the cozy, rustic flavors.