Add soup to sauteed onions. Microwave, uncovered, on high for 1-1/2 to 2 minutes, whisking occasionally. Continue cooking, stirring, until smooth and just bubbly. In a large skillet over medium-low heat, cook onion and celery in 4 tablespoons of butter until tender, about 5 minutes. Heat the oven to 400 F (200 C/Gas 6). Stir in flour, pepper, and dry mustard. Butter a 2-quart baking dish.

Stir in the broccoli, tuna, milk, salt and lemon-pepper; bring to a boil. Reduce heat; simmer for 5 minutes or until broccoli is tender. In a microwave-safe dish, whisk together water and soup mix. Stir in rice.