Melt butter in heavy large pot over medium heat. Add mushrooms, celery, carrots and onion. Cook until celery and onion are tender, about 5 minutes. Add flour and cook 3 minutes, stirring frequently. Gradually mix in chicken stock. Bring soup to simmer, stirring frequently. Add half and half and chopped parsley and simmer 5 minutes. Add chicken tenders and simmer until cooked through, about 5 minutes. (Chicken soup can be prepared 1 day ahead. Cool slightly. Cover and refrigerate.) Cook penne pasta in large pot of boiling salted water until just tender but still firm to bite. Drain pasta. Bring chicken soup to simmer. Mix in cooked pasta and sugar snap peas and simmer 2 minutes. Mix in lemon juice; season to taste with salt and pepper. Makes 12 First-Course or 6 Main-Course servings. Bon Appetit November 1999 Per serving: 1333 Calories (kcal); 77g Total Fat; (51% calories from fat); 31g Protein; 134g Carbohydrate; 227mg Cholesterol; 659mg Sodium Food Exchanges: 5 1/2 Grain(Starch); 0 Lean Meat; 6 Vegetable; 1/2 Fruit; 15 1/2 Fat; 0 Other Carbohydrates Converted by MM_Buster v2.0n.