In a medium mixing bowl, dissolve the brown sugar in the hot water. Stir in 1/2 cup of vinegar, and add red onion. Let sit for at least 30 minutes, while lentils cook.
Place lentils in a medium saucepan with crushed garlic and bay leaves, and cover with 2 inches of cold water (or for an added boost of flavor, a combination of water and chicken or vegetable broth). Bring to a lively simmer, and cook uncovered, for 20-25 minutes, until lentils are tender to the bite yet still hold their shape. Strain, remove bay leaves, and transfer to a large mixing bowl.
While lentils are cooking, toast the coriander and cumin seeds in a small skillet over medium heat for about 2 minutes, shaking frequently, until fragrant. Grind in a spice or coffee grinder, and transfer to a blender. Add remaining 2 tablespoons vinegar, lemon zest, lemon juice, tahini, olive oil, grated garlic and chile powder to the blender, and blend until smooth. Season to taste with salt and pepper.
Drain the onions, and toss with chickpeas, rosemary and lentils. Drizzle with the dressing, and toss well to coat. Toss again with parsley.