Heat oil in a large skillet over medium heat. Add onion, thyme, paprika, cayenne and kosher salt; cook, stirring, until the onion is translucent, 1 to 2 minutes.
Add lentils; cook, stirring frequently, until the onion is soft and the lentils are toasted and fragrant, about 2 minutes more.
Add 1 cup broth and 3 cups water. Reduce heat to medium-low and simmer, partially covered, for 15 to 20 minutes.
Uncover and simmer until the lentils are tender and most of the water is absorbed, 10 to 15 minutes more.
Meanwhile, bring the remaining 2 cups broth to a light boil in a medium pot. Add quinoa and cauliflower; reduce heat to medium-low, cover and cook until fluffy and tender, 12 to 15 minutes.
Mix yogurt, lemon zest, lemon juice and flaky salt in a small bowl.
Divide the quinoa mixture among 4 bowls. Top with the braised lentils, yogurt sauce and scallion. Garnish with a pinch of cayenne, if desired.
