Cut fennel bulbs into thin wedges. Halve wedges crosswise. In a very large kettle or 6- to 8-quart Dutch oven cook prepared fennel in 4 cups of boiling salted water, covered, for 3 minutes. Add loose-pack frozen vegetables and sliced celery. Return to boiling; reduce heat. Cover and simmer for 5 to 7 minutes or until vegetables are tender. Drain.
Meanwhile, in a small saucepan or skillet cook sesame seed in hot olive oil and margarine or butter for 1 to 2 minutes or just until seeds are light brown. Remove from heat. Carefully add lemon juice, salt, and pepper. Pour sesame mixture over drained vegetable mixture. Gently stir until coated. Makes 20 side-dish servings.