Prepare the zucchini: Cut 3.5 lbs (1.6 kg) zucchini into 1/4-inch (7mm) slices. Next, brown the zucchini in a pan. Heat the oil in a big frying pan over high heat. When hot, add the zucchini slices in one layer and cook briefly only until browned, flip over using two forks, and brown the slices on the other side. The zucchini doesn't have to be tender and cooked through, just browned. Transfer the zucchini into a plate lined with paper towels and repeat with the remaining zucchini slices. I usually cook the zucchini simultaneously in two frying pans. Pat the zucchini with paper towels to remove the excess fat. Place the zucchini in a large oval baking dish (9x13-inch / 23x33cm).
Preheat the oven to 375°F / 190°C / Gas Mark 5 (no fan, if you have a convection oven reduce the temperature by about 20° or follow the manufacturer's instructions).
Make the sauce: wipe excess oil from the pan and add 2 Tbsp of butter. When it's bubbling add 5 finely chopped garlic cloves and cook for about 30 seconds over medium heat, until fragrant. Add 1.5 Tbsp of flour and cook, stirring, for about a minute. Add all the other ingredients for the sauce - 1/2 cup cream, 1/2 cup milk, 1 ts of mustard, and 1 Tbsp of lemon juice. Cook for about a minute. Season the sauce to taste with salt and pepper.
Make the breadcrumb topping: Melt 3 Tbsp of butter in a small pot, take the pot off the heat. Stir in 1/2 cup of breadcrumbs until combined, then 1/2 cup of grated Parmesan.
Assemble the dish: Add the sauce to the zucchini and stir together. Top with 1 cup shredded cheese, then with the breadcrumb topping.
Bake for about 30 minutes or until the breadcrumb topping is golden.
Let the casserole rest for about 10 minutes before cutting into servings.