Place the tuna steaks into a large bowl. Zest the lemon over the tuna steaks. Cut the lemon in half and squeeze its juice over the tuna. Add the half of the salt, pepper and olive oil. Sprinkle over the dill weed.
Shake the steaks around to coat them evenly. You can also use a pair of tongs to turn them to ensure they are properly coated.
Line a baking pan with foil and fold the sides up so that none of the oil can leak out.
Place the steaks into the baking pan and spoon the oil mixture leftover on top of the tuna steaks.
Line a baking sheet with foil.
Cut the woody ends of the asparagus off and put them onto the baking sheet. Add the onions to the asparagus and drizzle with the remaining olive oil. Add the rest of the salt, pepper and all of the garlic powder. Evenly distribute the parmesan cheese over the asparagus and onions. Shake the baking sheet to coat everything well.
Place both the baking sheet with asparagus and the baking pan with tuna steaks into the oven. For a medium well tuna steak, you will need to cook it for about 10-12 minutes. (*if your steaks are thicker than one inch, you will need to cook it for longer to achieve medium well).
Test the asparagus with a fork, prodding it to see if they have softened.