Preheat the oven to 350ºF/180ºC.

Slice the onions in half. There's no need to peel them, unless you prefer to, at this point.

Place the fresh thyme into a large oven-safe dish or rimmed baking sheet (by placing it UNDER the onions, it won't burn in the process). Then drizzle olive oil over it.

Spread the onion halves, cut-side down, and sprinkle the entire dish with a bit of salt.

Then roast in the oven for 40-45 minutes, until the onions are tender and wonderfully caramelized/golden brown.

The exact baking time will vary based on the size of the onions you're using, so keep an eye on them. You can check if they’re tender with the tip of a sharp knife.

For quartered onions/onion 'wedges': the method is similar to the above, but peel them first if preferred. Then roast for 30-35 minutes. Flip halfway, so both cut sides have contact with the pan and air.

Whole roasted onions: peel most of the layers of papery skin from the onions except the final one. Cut off the very end of the root end and about ½ inch/1 cm from the other side, then drizzle with the oil/seasonings and bake for 60-80 minutes (the time will vary based on the size of the onions).

If the onions brown too quickly, you can cover the dish with foil.

Allow the roasted onions to cool, peel them, transfer any leftovers to an airtight container/bag, and store them in the refrigerator for 3-5 days.

When properly stored, cooked onions can last almost indefinitely in the freezer. However, I recommend using them within 10-12 months. Start by flash freezing them until solid, then transfer them to a freezer-safe container/Ziplock style bag (squeezing out excess air).

Allow them to thaw in the refrigerator overnight before enjoying/reheating. They'll last an additional 2-3 days after thawing.

You could reheat the onions in the microwave or chop them up and reheat them on the stovetop.