Preheat oven to 375° F
Line 2 cookie sheets with parchment paper. Pour bag of frozen vegetables onto the trays and bake for 45 minutes, until water is evaporated and vegetables are getting brown at the edges. Allow to cool.
Place 1-2 cups of veggies at a time in a food processor and pulse twice. This is just to break up the veggies; you don’t want them to become puréed. This step can be skipped, but it helps the casserole hold together and gives it a more uniform texture.
Sauté onion until golden, then add chopped or sliced mushrooms and sauté until soft. Combine onions and mushrooms with the vegetable medley. Season with salt and pepper. Add flaked tuna, eggs and cheese (if using) and mix well.
Pour mixture into a square or round glass casserole dish. Bake at 375°F for 1 hour and 15 minutes, or until very brown on top. Enjoy at room temperature, or rewarm uncovered.