Preheat oven to 425 degrees. Cut squash in half lengthwise. Scoop out seeds with a spoon and discard. Slice into 1-inch thick slices and arrange in a single layer on a large baking sheet. Drizzle with olive oil and flip to coat. Sprinkle with brown sugar, salt, and pepper. Arrange onion quarters and bacon slices around the sliced squash. Roast 30 minutes, until squash is fork-tender. When cool enough to handle, remove bacon and onion to a cutting board. Chop the roasted onion. Crumble the bacon. Using your fingers, separate the squash from the rind. Discard the rind and place squash in the bowl of a stand mixer fitted with the whisk attachment along with the chopped onion and cream cheese. Mix until creamy. Adjust seasoning to taste. Melt butter in a large saucepan or griddle over medium-high heat. Grill sliced baguette until golden brown. Remove and rub with garlic. To assemble bruschettas, spread each piece of grilled baguette with squash mixture and top with crumbled bacon. Serve warm or at room temperature.
You can make this topping ahead. Just prepare per the directions, store the squash mixture and bacon in separate airtight containers, and reheat squash prior to serving. I'd be sure to use it within 3 days of making it.