Preheat the oven to 400°F (204°C).
For the cucumber water, place the cucumber slices in a pitcher with water. Let infuse in the refrigerator. Serve over ice if desired.
Slice off the top portion of the garlic head to expose some of the cloves. Place the garlic on a piece of foil. Drizzle lightly with olive oil and wrap in foil. Transfer to the oven and roast until cloves are lightly browned and tender, about 20 minutes. Let cool, then squeeze the garlic out into a bowl. Mash the cloves with a fork.
To make the meat sauce, heat the olive oil in a large pot over high heat. When the oil shimmers, add the sausage and brown all over, about 5 to 8 minutes. Add the chopped onion and cook for another 2 minutes. Reduce the heat to low, add the tomatoes, water, the bay leaf, oregano, and a pinch each of salt and pepper. Simmer for 15 minutes or until the sauce thickens slightly. Remove the bay leaf; stir in the roasted garlic. Taste and adjust seasonings as needed.
Meanwhile, poke a few holes into the squash with a fork and place on a plate. Microwave for 3 minutes. Let cool slightly, then cut in half lengthwise; scoop out the seeds and discard.
In a medium bowl, mix together the milk, ricotta cheese, nutmeg, and basil. Season with salt and pepper. Stir in fontina cheese and provolone cheese.
Place the squash halves in a baking dish and season the cut sides with salt and pepper. Sprinkle the remaining fontina cheese into the bottom of each squash, then evenly divide the meat sauce among the squash cavities. Now spoon the milk/cheese mixture over the meat sauce. Top with the remaining provolone cheese. Cover the squash loosely with foil.
Transfer to the oven and bake for 20 minutes. Remove the foil and continue baking for another 15-20 minutes or until the squash is tender and the cheese is golden brown on top. The squash will seem a little soupy, but this is normal.
While the squash is baking, prepare the house salad. In a large bowl, combine the mixed greens, cherry tomatoes, cucumber, and red onion. Toss with balsamic vinaigrette dressing, or reserve dressing to keep extra servings of salad fresh.
Let the squash sit for 5 minutes, then use a fork to scrape the squash into strands, mixing the meat and cheese with the squash.
Serve the roasted garlic spaghetti squash lasagna boats alongside the house salad and cucumber water.