Add the chopped onion, carrot, jalapeno and garlic to a food processor. Pulse until finely chopped, for about 10 seconds, scraping down the sides half way through. Alternatively, you can finely chop the vegetables by hand if you don't have a food processor. Using a fine mesh strainer, strain excess liquid from vegetables before adding to skillet. (Note: this step isn't necessary if you choose to chop the vegetables by hand.)
In a large skillet, warm 2 tablespoons olive oil over medium heat. Add the finely chopped onion mixture and cook for about 5 minutes. Reduce heat to low.
Meanwhile,add the walnuts to the processor and pulse until finely chopped. Transfer to the skillet. Next add half of the lentils to the food processor and pulse a few times, until slightly mashed. Scrape down the sides in between pulsing. You can also mash them by hand using a fork or a potato masher. Scoop them into the skillet along with the remaining lentils, tomato paste and seasonings. Stir to combine as you cook over low heat.
Continue to cook until some of the moisture has absorbed, so that is doesn't have a mushy texture, usually about 5-10 minutes. Serve warm with tortillas and desired toppings, and enjoy!