Position one rack in the center of your oven and a second rack in the top third and preheat to 400 degrees F. Line two sheet pans with parchment paper.
Divide the squash evenly between the prepared pans, then drizzle each pan with 2 Tbsp of the olive oil and season generously with salt and pepper. Use your hands to toss everything.
Roast the squash, stirring once or twice along the way and switching the pans between the racks and rotating them back to front halfway through the roasting, until softened and browned, about 30 minutes. Set the squash aside to cool down a bit.
In a large bowl, whisk together the mustard, honey, vinegar, and garlic. While whisking constantly, slowly, drizzle in the remaining 1/2 cup olive oil to make a dressing. Season to taste with salt and pepper. Add the reserved squash and the radicchio to the bowl and use your hands to combine everything gently (messy, but fun).
Transfer the salad to a serving platter and serve immediately ( or withing a few hours; it holds well at room temperature.