Preheat oven to 400 degrees
Using a spoon, scoop centers from zucchini to create boats while leaving a 1/4-inch rim (reserve centers for another use)
Brush both sides of zucchini with 1 Tbsp of the olive oil and season lightly with salt.
Place in two baking pans (I used a 13 by 9 and 8 by 8).
Bake in preheated oven 20 -25 minutes until nearly tender.
Meanwhile, heat 1 Tbsp olive oil in a large non-stick skillet over medium-high heat. Add onion and sauté 5 minutes, then add another1/2 Tbsp olive oil and mushrooms and sauté 5 minutes longer.
Push veggies to one side of pan then add remaining 1/2 Tbsp olive oil to opposite side, add bell pepper and garlic and sauté until tender, about 3 minutes.
Add in roast beef, season with salt and pepper to taste then stir in water and cook until warmed, about 30 seconds.
Fill roasted zucchini with roast beef mixture. Sprinkle top evenly with cheese. Return to oven and bake about 5 minutes longer, or until cheese has melted.
Serve warm garnished with parsley.(fyi these are easier to just pick up and eat by hand like a sandwich).