For the sauce, heat a Dutch oven or saucepot over medium to medium-high heat with oil. Add onions, garlic, red pepper flakes and some salt, and cook to soften 10 minutes. Add tomatoes and passata or purée, and simmer 20-25 minutes to break down tomatoes. Process with a stick blender or in a food processor; stir in basil.
Preheat oven to 450°F.
While the sauce simmers, trim zucchini ends and cut them in half lengthwise. Stand them up on one end and slice each zucchini into very thin planks. Salt zucchini and let it drain on towels for 30 minutes.
Line 2 baking sheets with parchment paper and arrange the zucchini in a single layer. Spray with olive oil spray then roast to just tender and browned, 12-15 minutes, moving trays about midway through. Remove from oven and cool to handle. Reduce heat to 375°F.
Line a rectangular baking pan with a ladleful or two of sauce. Add a layer of sliced roasted zucchini, then more sauce and some Parm. Repeat with remaining ingredients to create a few more layers. Top with mozzarella and a sprinkle of Parm. Bake 30-35 minutes to bubbly and brown; remove from oven and let stand a few minutes to settle.
For the garlic bread, lightly toast bread on a baking sheet. While bread toasts (keep an eye on it to make sure it doesn’t burn!), in a small pot, heat EVOO and melt butter into it. Add garlic and swirl 2 minutes. Brush the garlic butter on toasted bread and top with cheese and parsley. Return to oven 2-3 minutes until edges are brown.
Cut Zucchini Parm into squares and serve in shallow bowls with hunks of garlic bread alongside.