How to Make It
Toast cumin and mustard seeds in a large, dry saucepan over medium, stirring often, until fragrant, about 30 seconds. Remove from heat, and grind into a coarse powder. Add oil to pan, and heat over medium-high. Add cumin mixture, chiles de árbol, and bay leaves, and cook, stirring constantly, until fragrant, about 25 seconds. Reduce heat to medium, and stir in onion. Cook, stirring occasionally, until onion softens, about 15 minutes. Stir in garlic, and cook, stirring often, until fragrant, about 2 minutes. Stir in salt, paprika, and turmeric.
Whisk together 1/2 cup water and chickpea flour in a small bowl until well combined. Add chickpea flour mixture, broth, tamarind water, lentils, and potato to pan. Stir to combine, and bring a boil over medium-high. Reduce heat to medium-low, and simmer until lentils and potato are tender, about 20 minutes. Stir in Thai chiles and garam masala.
Garnish servings with chopped mint, sliced cabbage, cilantro leaves, sliced Thai chiles, and lime wedges.
To make tamarind water, combine 3/4 cup boiling water and 1 heaping tablespoon tamarind pulp. Let steep 1 minute. Mash with a fork, and let steep 3 minutes. Pour through a fine wire-mesh strainer into a bowl; discard solids.