Heat 2 tbsp olive oil in a large pan over medium-low heat. Add mushrooms, salt, and pepper, and cook until brown and fragrant, stirring occasionally, about 10 minutes. Place mushrooms on a plate and set aside.
In the same saucepan over medium heat, add the shallots and garlic and cook for 5-6 minutes or until softened. If using pre-cooked lentils, throw them into the pot now and skip to step 4.
If making your own lentils, rinse 1⁄2 cup of lentils in a mesh colander, drain well, transfer to a saucepan, and pour in 1 cup of water. Bring the water to a rapid simmer over medium-high heat, then reduce heat to a very gentle simmer. Cook, uncovered, for 20-30 minutes.
Pour the other 2 tbsp olive oil into the pan with the shallots and garlic and reduce heat to very low. Add the cooked lentils, cumin, paprika and a sprinkle each of salt and pepper. Stir well to coat the lentils with spices. Add the mushrooms back to the pan and mix to combine.
Warm the tortillas over the stove top or in the microwave wrapped in a kitchen towel to keep them soft. Spread a spoonful of the tahini sauce, the mushroom lentil mixture, arugula and another dollop of sauce.
Notes
If you buy pre-cooked lentils and make the tahini sauce in advance, this recipe should take you all of 10 minutes. These tacos are full of flavor and completely plant-based!