Prep: Preheat the oven to 425F and line or grease a baking sheet. Sort through and rinse the lentils, then add them to a large pot and cover with water (1-2" above the lentils). Add 1 teaspoon of garlic powder and a generous pinch of salt to the lentils, then stir to combine. Bring to a boil over high heat, then reduce heat to low and simmer, covered, for 25-30 minutes, until the lentils are tender. Drain well, then set aside.
Roast the Cauliflower: In the meantime, add the cauliflower to a large bowl. Drizzle with olive oil, then add the curry powder, garlic powder, cayenne (optional), and salt. Toss the cauliflower until all pieces are evenly coated, then transfer to the baking tray. Bake in the top rack of the oven for 30-35 minutes, until the edges are browned.
Make the Dressing: prep the remaining vegetables for the salad. Whisk the oil, vinegar, mustard, curry powder, and salt together in a small bowl until well-combined; set aside.
Assemble: Add the drained lentils, roasted cauliflower, carrot, pickled onions, and cilantro to a large bowl. Pour the dressing over the bowl, then toss until well-combined. Season with additional salt to taste, if necessary.
Serve as desired; leftovers will keep in the fridge for up to 5 days and can be served hot or cold.