Pulse the walnuts in a food processor until finely minced (resembling bread crumbs).
Heat the oil in a skillet over medium heat. Add the onion and garlic, sauté until softened (about 3-5 minutes).
Stir in the tomato paste, cumin, coriander, and chili flakes if using. Cook for 1 minute more.
Add lentils, chopped walnuts, and tamari. Stir well and cook 5-7 minutes until the mixture is warmed through and slightly crisp. Sprinkle on salt & pepper, to taste.
Make the Tahini Lime Sauce
In a small bowl, whisk together the tahini, lime juice, maple syrup, garlic, and salt.
Add the water 1 tablespoon at a time, whisking until the sauce is smooth and pourable (like a drizzle).
Taste and adjust. Add more lime juice, maple syrup, or salt to balance the flavors.
Assemble the Tacos
Warm the tortillas, if desired. Fill each tortilla with the lentil-walnut taco "meat", shredded purple cabbage, avocado slices, cilantro, and a large drizzle of the Tahini Lime Sauce.
Enjoy with a non-starchy vegetable side such as riced cauliflower, roasted broccoli, or steamed carrots to round out the meal!