Step 1: In a large skillet or Dutch oven, heat olive oil over medium heat. Add the diced onion, carrots, and celery. Sauté for 8-10 minutes, stirring occasionally, until softened and lightly golden.
Step 2: Add the minced garlic and tomato paste to the skillet. Cook for another 2-3 minutes, stirring constantly, until the tomato paste darkens slightly and becomes fragrant.
Step 3: Transfer the sautéed vegetables and tomato paste mixture to your slow cooker. Add the rinsed brown or green lentils, crushed tomatoes, vegetable broth, dried oregano, dried basil, and bay leaf. Stir everything together until well combined.
Step 4: Season generously with salt and black pepper. Cover the slow cooker and cook on low for 6-8 hours, or on high for 3-4 hours, until the lentils are tender and the sauce has thickened. Stir occasionally if possible, but it’s not essential.
Step 5: Once cooked, carefully remove and discard the bay leaf. Stir in the nutritional yeast (if using) and balsamic vinegar (if using). Taste and adjust seasonings as needed, adding more salt, pepper, or a pinch of sugar if the tomatoes are too acidic.
Step 6: Serve the Slow Cooker Lentil Bolognese hot over your favorite pasta, polenta, mashed potatoes, or zucchini noodles. Garnish with fresh chopped parsley and vegan Parmesan cheese, if desired. Enjoy your comforting, plant-based meal!