Increase oven to 375°
Place rice in a sauce pan and add water plus a pinch of salt. Bring to a gentle simmer, cover and let cook 5 minutes. Turn off heat and allow to sit, covered, 5 more minutes. After 5 minutes, uncover and stir. set aside until ready to add to filling. Rice will not be cooked through and will not have absorbed all of the water. This is ok, we're just partially cooking it. It will finish when it's baked in the oven.
In a wide skillet heat 1 Tablespoon oil. Add diced onion and saute 1 minute, just to soften. Crumble ground beef up into skillet along with minced garlic. Stir and cook, breaking up ground beef, until almost cooked through, about 3-4 minutes. You still want to see some pink in the meat.
Turn off heat.
To the pan add rice (don't drain if it's a little wet), ketchup, drained tomatoes, 3 Tablespoons diced pickles, yellow mustard, worcestershire sauce, kosher salt and cracked pepper to taste (I use about 3/4 teaspoon kosher salt and maybe 8 twists of cracked pepper). Stir well and then add most of the cheese (reserving enough to sprinkle tops of zucchini liberally once stuffed).