Note -- Potatoes -- Well, you can use some leftover mashed potatoes, you can even buy deli mashed or even the frozen mashed. You could always use even Hungry Jack boxed for this although that would be my last choice.
Personally I would peel a couple of russet potatoes, dice and put in a bowl with a teaspoon of water, cover with plastic wrap and microwave for 5 minutes until soft. Mash and add some milk/cream/broth and some butter and you have fresh potatoes. Just make 2 cups and set to the side in a medium size bowl.
Zucchini -- Cut the ends off and cut lengthwise. Scoop out the seeds and discard them. Then scoop out some of the pulp, and keep about 8 tablespoons to mix with the potatoes. Make sure to scoop out enough pulp so you can stuff with the filling. If the the zucchini doesn't sit flat, just cut a little piece off the bottom so it can set flat. Add the 8 tablespoons of the zucchini pulp to the potatoes.
Bacon -- In a small saute pan, cook the bacon until golden brown. Remove to a paper towel to drain and set to the side and then add to the potatoes and zucchini mix.
Onion -- In the same pan with 1 teaspoon of the bacon drippings, cook the onion and garlic until lightly brown, just a couple of minutes. Remove and add that to the bowl with the potatoes, zucchini, and bacon.
Stuffing -- Mix the the potatoes, zucchini, onion and bacon well, add any salt and pepper if necessary and add the parsley and mozzarella cheese. Stuff each zucchini half.
Topping -- To seed the tomatoes, simple cut the plum tomatoes in half and then squeeze. Most of seeds should just fall out. Then chop. This way you get rid of the bitter seeds and most of the juice.
Add the tomatoes, bread crumbs, parmesan cheese and olive oil in a small bowl until well combined. Top each zucchini with the breadcrumb, tomato and cheese mixture.
Bake at 375 for 30-40 minutes until the zucchini is tender and the bread crumbs are golden brown.