Garlic: Remove excess papery skin from the garlic heads. Chop the top off of each head so that the tops of most cloves are exposed. Evenly pour 1 Tbsp of the olive oil over each head so that it seeps into the cloves.
Veggies: Chop all the veggies into bite-sized pieces. Combine veggies in a large bowl with remaining 2 Tbsp of oil and all seasonings, tossing to evenly coat.
Assemble: Evenly divide the veggies and garlic heads into 2 disposable aluminum pans (8 x 8 inch). Cover tightly with aluminum foil and store in a fridge or cooler until ready to cook (up to 3 days).
Cook: Cook veggies over hot coals, or elevated over a fire, until potatoes are fork tender, about 30 to 60 minutes.
Notes
If storing in a cooler, place the pan in a gallon-sized ziploc bag to prevent water from getting in!