Lentil Curry

olive oil

medium onion, finely chopped

curry powder

sweet potatoes, peeled and cut into 1-inch chunks

head cauliflower (2 1/2 pounds), stemmed and separated into florets

brown lentils, picked over, well rinsed, and drained

each) diced tomatoes with juice

Coarse salt

Plain low-fat yogurt, for serving (optional)