To a food processor, add your cooked and thoroughly drained lentils, along with the sautéed mushroom mixture. Add in the remaining ingredients: sunflower seeds, soy sauce, paprika, and black pepper to preference. Process until mixture is well-combined but still has a little bit of texture. Scrape down the bowl of the food processor as needed to ensure the texture is consistent.
Add breadcrumbs and stir to combine. (You can do this directly in your food processor if there is room, or transfer to a bowl if you need extra space to mix.) Stir to evenly distribute the breadcrumbs.
The mixture should be moist enough that it sticks together and doesn't crumble. However, it shouldn't be so wet that it sticks to your hands. If the mixture is still a little on the wet side, add in additional breadcrumbs a few tablespoons at a time and mix well between additions. Go by feel — the exact amount you need will depend heavily on the texture of your cooked lentils, and how much moisture you cooked out of your mushrooms.