Make a small slice in the peppers and remove seeds. Set aside on paper toweling to dry.
Heat the vegetable oil in a skillet and saute zucchini, squash, eggplant, onion, and garlic until tender. Remove from heat and fold in bread crumbs and cheese, and set aside to let cool.
Mix the flour, cornstarch, and baking powder in a bowl and add seltzer in a stream while whisking constantly. Season with salt and pepper.
Heat canola oil to 375 degrees F in a deep fryer.
Stuff each pepper with a small amount of the vegetable stuffing. Then dip each stuffed pepper in the tempura coating and deep-fry until golden brown. Drain on paper toweling and serve warm.