Here's the recipe I use, when I don't have any tahini.
1 cup garbanzo beans, soaked and cooked
1 tbsp. EVOO or sesame oil
2 tbsps. lemon juice
2 garlic cloves, minced
1/4 cup water (or veggie or chicken stock -- really gives it good flavor)
1 tsp. horseradish
Process all ingredients until smooth. Easy, peasy.
ETA: That's adapted from the recipe in Eat to Live, btw.