Zippy Yams and Bok Choy

Makes 4 servings

2 small yams, cut into bite-size chunks
1 onion, quartered and sliced
2 large garlic cloves, minced
1 tablespoon vegetarian Worcestershire sauce
1/2 teaspoon Thai chili paste
2 small heads bok choy, finely sliced
juice of 1/2 lemon