Heat large, heavy saucepan over medium-low heat. Add onion and curry paste, stirring paste thoroughly into onion; cook, stirring constantly, for 2 to 3 minutes.

Smash cardamom pods with large knife or meat mallet. Remove small brown seeds (discard pods) and stir into onion mixture along with sweet potatoes; cook for 2 to 3 minutes.

Stir in lentils and broth; cover and bring to boil over high heat. Reduce heat and simmer, covered, for 15 minutes.

Add cauliflower and return to boil; reduce heat and simmer for 8 to 10 minutes or until lentils and vegetables are tender.

Remove from heat; stir in cilantro. Season to taste with salt and pepper. Let stand a few minutes before serving with a dollop of yogurt if desired.