HEAT a pot over medium-high. Add 1 tbsp oil, then onions and salt. Cook, stirring often, until almost soft, about 6 min. Reduce heat to medium. Continue cooking, stirring often, until golden brown, 9 to 10 more min. Add 1 to 2 more tbsp of water if bottom starts to brown. Transfer to a plate.
ADD remaining 1 tbsp oil to same pan. Stir in garlic, cumin and cinnamon. Cook for 30 sec. Stir in broth and lentils. Boil, then reduce heat to medium-low and simmer, covered, 10 min. Stir in rice and continue simmering until lentils are tender and liquid has been absorbed, 20 more min.
STIR in half of the prepared onions. Transfer to a platter.
Top with remaining onions. Drizzle with yogurt and sprinkle with mint.