Prep onion and lentils. Mash lentils in a medium bowl, then add all remaining cake ingredients (except oil) and mix well to combine. Use a ¼ cup measure to shape mixture into patties, about 1.5cm thick.
Heat oil in a medium fry-pan on medium-high heat and cook lentil cakes, for 2-3 minutes each side, until brown and cooked through. Set aside to drain on a paper towel.
Prep capsicum and toss in a medium bowl with slaw, soy sauce and first measure of lime juice. Season to taste with salt and pepper. In a small bowl, stir together mayonnaise and remaining lime juice.
Serve slaw topped with lentil cakes, mayonnaise and peanuts.