Heat oven to 400°F. Slice the top from the garlic head to expose the cloves, drizzle with oil, wrap in foil and roast for 40 to 45 minutes until soft; cool for 10 minutes.
Meanwhile, toss cabbage with vinegar, sugar and its measured salt. Stand for 20 minutes, turning once, then drain thoroughly and squeeze lightly.
Squeeze roasted garlic from the skins into a food processor. Add beans, yogurt, lemon juice, spread salt and pepper and blend until smooth, scraping the bowl as needed.
Toast only the cut bagel faces under the broiler for 1 to 2 minutes and let cool. Spread the garlic-bean mixture evenly over both cut faces.
Fold roast beef loosely over the four bottoms and divide drained cabbage over it. Close with the tops, cut in half and serve cold, keeping unused sandwiches refrigerated.