Place the lentils into a 3-quart pot and cover with water by two or more inches. Add the minced onion. Place the pot over medium heat. Slowly bring the lentils to a boil then reduce the heat to a simmer and cook the lentils until tender adding a pinch or two of salt in the last 10 minutes of cooking. This should take approximately 30 minutes.
Drain the lentils. Let them cool a bit, then puree them in a food processor while they are still warm. They will be easier to handle when warm.
Add the remaining lentil cake ingredients and pulse the cakes a few more times until the rest of the ingredients are combined into the mix. Taste the lentil puree then season the puree with kosher salt and a few grinds of pepper. Taste again and adjust the seasoning.
Let the cakes sit for a few minutes to hydrate the flour. Take a tablespoon of the mix and make a ball. Is it really wet or is it too stiff? You want the mix to hold its shape but not be overly stiff otherwise they can be dry when cooked. It should just hold its shape.
Add more flour a tablespoon at a time if you need to letting the additional flour hydrate before testing. Divide the lentils into eight balls.
Add enough oil to cover the bottom of a heavy bottomed sauté pan by an 1/8 inch. Heat the oil over medium high heat. Test the oil by dropping a pinch of lentil to the pan. It should begin to sizzle right away but not violently sizzle and pop.
When the oil is ready take each lentil ball and smash it down gently forming it into 1/2 inch thick cakes and add them to the oil. Let each side brown nicely and then remove them to a tray lined with a brown bag to soak up the oil. Keep the cakes warm, either in a low, 200° F oven or in a warm place on the stove.
Drain the oil from the pan, place it back on the heat and then add the remaining diced onion. Sauté until tender then add the rest of the sauce ingredients. Stir to combine, bring to a boil then reduce the heat. Let it simmer for ten minutes to come together. You can puree the sauce to make it smooth or leave the onion chunky making the sauce rustic.