The cumin and mustard seeds are cooked in very hot oil until they pop; this classic Indian cooking technique releases their flavors.
Toast cumin seeds in a small, dry skillet over medium-low heat until aromatic, about 2 minutes, shaking pan to avoid burning. Set aside.
Heat oil in a large saute pan over medium-low heat. When oil is hot, add mustard and cumin seeds; after a few seconds seeds will pop. Immediately add chiles and sliced cauliflower; saute, stirring often to keep from burning, until just tender and golden brown around the edges, about 8 minutes. The cauliflower should be al dente, tender but with some texture, not soft.
Stir in chopped cilantro just before removing mixture from pan. Drizzle with lemon juice; season with salt and pepper. Garnish with chile slices and cilantro leaves. The saute is best served right away.