Preheat the oven to 425ºF. Remove the outer layers of the garlic head and then slice off a thin layer off the top and sides to expose the skin of all of the cloves. Rub the exposed cloves with a bit of olive oil. Wrap in aluminum foil to make a packet.*
Remove any of the dark loose outer leaves on the cabbage and slice off the bottom nub. Slice the cabbage to get six or eight 3/4” thick steak-style slices. With any remaining cabbage, slice into wedges (or steak-style cuts, it’s up to you).
Place the cabbage steaks on a rimmed sheet pan and drizzle the first side with 1 ½ to 2 tablespoons of olive oil, and rub it in. Season generously with salt and pepper. Flip and repeat with the oil and salt and pepper on the second side. Do the same with the cabbage wedges and onion wedges.
Spread out the cabbage and onions across two sheet pans. Add the garlic packet on one of the pans.
Roast for 20 to 25 minutes, until there is some browning on the top of the cabbage steaks. Carefully flip the cabbage steaks to avoid breaking. The cabbage steaks will need another 15-20 minutes in the oven, depending on their size. If the outer leaves are browning, don’t worry - they are so tasty! The cabbage wedges, onion wedges, and garlic should only need another 10ish minutes. The garlic is done when cloves are very soft and practically oozing out of their skins
Store cooled vegetables in an airtight container for 4 to 5 days. If desired, stack cabbage steaks on top of each other in a wide container to retain their shape. To reheat cabbage steaks, add them to a baking sheet and roast at 400ºF for 10-15 minutes until heated through.
*Or, you can wrap the garlic directly in parchment paper and then wrap in foil. The garlic will take about 10 extra minutes to roast this way.