Rinse and drain lentils. Add to a large saucepan along with rice.
Add water to the saucepan until lentils are covered by about 3 inches. Bring to a boil, cover, then reduce heat and simmer for 20 minutes until the lentils and rice are softened.
Drain lentil/rice mixture, then add to a food processor; pulse until finely chopped. Leave in the processor and set aside.
Heat 1 tablespoon oil in a large skillet over medium.
Chop green onion then add to the hot oil. Cook for 2 minutes, then add tomato paste and red pepper flakes. Cook for 1 minute, stirring frequently.
Add onion mixture to the rice and lentils in the food processor, along with 2 tablespoons chopped cilantro, the lemon juice, and salt; blend to combine.
Divide the mixture into 8-12 equal portions. Roll each portion into a ball, then press into patties.
Heat 1 tablespoon of oil in another large skillet.
Add patties to the skillet, working in batches to avoid overcrowding.
Cook patties for 3 minutes per side until browned. Add more oil as needed, continuing to cook until all patties are done.
While patties cook, dice cucumbers and mangoes. Toss with remaining chopped cilantro and your favorite vinaigrette dressing.
Serve each patty on top of a large lettuce leaf, with cucumber-mango salad on the side. (Serving size: 2 patties, 3/4 cup salad)