Place the squash, cut-side down, in a 13-by-9-inch microwaveable dish; cover. Microwave on High until tender, 10 to 12 minutes. Cool slightly.
Meanwhile, cook the sausage and onions in large skillet over medium-high heat until thoroughly cooked, 8 to 10 minutes, stirring frequently; drain. Return the sausage and onions to the skillet. Add the breadcrumbs, spinach, cranberries, cheese, almonds, milk, and egg; mix well.
Turn the squash halves over; fill with the sausage mixture. Microwave, covered, until thoroughly heated, 2 to 3 minutes. Cut in half to serve.