In a medium pot, bring 3 cups of water to a boil. Add the rinsed lentils, reduce heat to low, and simmer for 20-25 minutes until tender but not mushy. Drain any excess water and set aside.
Stir in the minced garlic, chopped carrot, and celery. Cook for 5-6 minutes, stirring occasionally, until the vegetables begin to soften.
Add the tomato paste and cook for 1-2 minutes, stirring to coat the vegetables. Pour in the crushed tomatoes, vegetable broth, oregano, basil, and red pepper flakes (if using). Season with salt and black pepper. Stir well to combine.
Add the cooked lentils to the sauce. Bring to a simmer, then reduce the heat to low. Cover and cook for 15-20 minutes, stirring occasionally, until the sauce thickens and the flavors meld. Adjust seasoning to taste.
Serve the vegan lentil Bolognese hot over cooked spaghetti squash, zucchini noodles, or your favorite pasta alternative. Garnish with fresh parsley or basil if desired. Enjoy a hearty, plant-based twist on a classic!