Combine red onion, cucumber, vinegar, and lime juice in a small nonreactive bowl. Cover, and refrigerate for at least 2 hours (or up to overnight).
Heat oil in a saucepan over medium-high heat. Add yellow onion, garlic, and chile, and cook until onion is soft, about 4 minutes. Add spices and salt, and cook, stirring frequently, for 2 minutes.
Stir in tomato paste. Add lentils, potato, cauliflower, stock, and water. Bring to a boil. Reduce heat to low, cover, and simmer until lentils are tender but not breaking down, 15 to 20 minutes. (Check lentils after 10 minutes, adding water if necessary.) Remove from heat, and let stand, covered, for 10 minutes.
Serve lentil mixture over rice, and top with strained red-onion mixture. Garnish with lime wedges for squeezing.