Prepare the lentils: Heat the oil in a large skillet over medium heat. Add onion, garlic, and a pinch of salt and cook, stirring frequently, for 5 minutes. Add celery. Meanwhile, whisk together tomato paste, vegetable broth, and sugar or maple syrup in a bowl.
In a separate bowl, whisk together all of the spices, salt, and pepper. Add the tomato paste mixture to the skillet, followed by the spice mixture. Add lentils and stir to combine. Then reduce heat to medium-low. Simmer, stirring occasionally, until some of the liquid has absorbed/evaporated, about 15 minutes. Remove from heat, cover, and set aside until ready to use. This will allow the flavours to meld.
Prepare the sweet potatoes: Preheat oven to 400 degrees F. Place the diced sweet potatoes on a parchment paper-lined baking sheet. Drizzle with the olive oil and sprinkle with 1/4 tsp salt and pepper to taste. Toss to coat. Roast for 25-35 minutes, or until fork-tender and crispy around the edges.
While the sweet potatoes are roasting, you will have time to prepare the remaining components. To make the cabbage slaw, toss together cabbage, oil, lime juice, garlic, salt, and pepper in a medium bowl. Set aside until ready to use.
Make the Cilantro-Lime Crema: Blend cashews, cilantro, lime juice, nutritional yeast, garlic, salt, and water in a blender until smooth and a thick, sour cream-like consistency is achieved. If it seems too thick, add more water, 1 tbsp at a time.
Make the Quick Pickled Onions: Peel and thinly slice the onions. Place them in a 16-oz/500 mL glass jar. In a small saucepan, combine the vinegars, water, maple syrup, salt, and red pepper flakes over high heat. Bring to a boil, then remove from heat and pour over top of the onions in the jar. Make sure that the liquid is covering all of the onions. Secure a lid on the jar and set aside to pickle for at least 10 minutes, preferably longer (the onions will soften and their sharpness will mellow the longer you allow them to pickle). Make ahead: Onions will keep in the fridge for at least a week.
Warm up the tortillas in a covered dish in a 300 degree F oven for 5 minutes prior to eating, if desired.
To assemble a taco, spoon desired amount of lentils, followed by sweet potato, cabbage slaw, pickled onions, and cilantro-lime crema onto a tortilla. Repeat as desired.