Pick through and thoroughly rinse the lentils.
In a medium sauce pan, combine the lentils, sunflower seeds, and water over medium-high heat. Bring to a boil, reduce the heat, and simmer, uncovered, for 20 to 25 minutes, or until the lentils are tender but not mushy. Strain off any excess water but do not rinse. Transfer to a large, heat-safe mixing bowl, and use the back of a fork to coarsely mash about a third of the lentils. You still want them to have quite a bit of texture.
While the lentils and sunflower seeds cook, heat the olive oil over medium heat in a large sauté pan. Add the onion and garlic and sauté for 5 minutes or until translucent, stirring occasionally. Add the crushed tomatoes, sun-dried tomatoes, tomato paste, balsamic, oregano, basil, and red pepper flakes (if using). Cook for 5 to 8 minutes, or until most of the liquid has cooked off and out of the mixture, stirring occasionally. Remove from heat and let cool slightly.
Meanwhile, in a small bowl, whisk together the flaxseed and water. Let stand for 5 minutes to thicken.
Transfer the cooked tomato mixture to the mixing bowl with the lentils and sunflower seeds. Stir to combine. Add the flax egg, oat flour, rolled oats, and salt. Stir to combine. Taste and add more salt if desired.
Line a baking pan with parchment paper. Grease a 1/4-cup measuring cup with a little bit of oil, and use it to scoop up 1/4 cup of the slider mixture. Then, form into a 1-inch-thick patty, and place on baking pan. Repeat. I like to stop halfway through to rinse + re-oil the measuring cup and my hands. The recipe yields 10 to 12 patties.
Refrigerate the patties for 1 hour (or more) to set them before cooking.
While the patties are chilling, prepare the Basil Aioli—see recipe below. Refrigerate until ready to serve. Also, blend the ingredients for the Vegan Cashew Mozzarella (if using), but wait to heat/thicken until directed to do so below.
In a large skillet or sauté pan, heat the high-heat oil over medium-high until hot but not smoking. Add half of the slider patties to the pan, being careful not to crowd them. Cook for 3 minutes, or until golden brown, and then carefully flip over and cook for another 2 to 3 minutes or until golden brown. Transfer to a plate that has been lined with paper towels until ready to assemble. Repeat. You may need to add a bit more oil.
Heat/thicken the Vegan Cashew Mozzarella (if using) just before assembling.
To assemble the sliders, slice the buns in half, place a patty on top of each, dollop with the mozzarella (if using) and pizza sauce (if using), and drizzle with the aioli.