• Preheat the oven to 350℉
• Heat 1 tablespoon olive oil in a large skillet over medium heat. Add the onions and cook for approximately 3 minutes, until just softened. Add the garlic clove and cook for 1 additional minute. Remove the onions and garlic and place in a medium bowl.
• Return the pan to the heat and add the mushrooms. Cook until browned on both sides and glistening. The onions should not be watery. If so, cook a little while longer until the water evaporates. Place the mushrooms in the bowl with the onions.

Make the Sauce

• Return the skillet to the stove and add the remaining 4 tablespoons of olive oil to the pan. Heat over medium heat. Add the flour and whisk for 3 minutes until the flour and oil starts to bubble.
• Slowly add the vegetable broth, whisking while adding it. Continue whisking to get a creamy sauce without any lumps. Add the lemon juice, lemon zest, and 1/2 the dill. Continue whisking for another minute. Your sauce should be creamy and smooth.
  

Cook the Pasta and Broccoli

• Bring water to a boil in a large stock pot or saucepan. Add the pasta and cook according to the package directions. (About 10 - 12 minutes)

  When the pasta has 2 minutes left to boil, add the broccoli florets to the water to cook.

  Drain the pasta and broccoli and return it to the stockpot
  

Assemble the Pasta Bake

• Pour the creamy lemon dill sauce over the pasta and broccoli. Add the mushrooms and onions and mix well.
• Open the tuna and gently stir the tuna and any oil into the pasta mixture, being careful to not break up the tuna too much.
• Pour the pasta and sauce into a 9x13 casserole dish. Place it in the middle of the preheated oven and bake for about 20 minutes, until it is bubbling.
• Sprinkle with shredded mozzarella cheese and the remaining chopped dill, if desired.