Step 1

Preheat the oven to 400F. Line a large baking sheet with parchment paper

Step 2

Remove the stems from the mushrooms, and using a small spoon, carefully scrape off the gills, making sure the mushrooms don't break (you want to create a cavity for the filling)

Step 3

Brush each portobello mushroom top with some olive oil and place the stem side down on the  lined baking sheet. Wrap the unpeeled garlic cloves in aluminum foil and place them on the baking sheet next to the mushrooms. Bake for 12 minutes (15 minutes if the mushrooms are extra large)

Step 4

Remove the baking sheet from the oven. Carefully transfer the mushrooms to a large plate, discard any water that may have puddled on them and return the baking sheet to the oven to continue cooking the garlic cloves for an additional 15 minutes.  Do not turn off the oven after you take out the garlic

Step 5

While the mushrooms bake, peel the potatoes, cut them into 2" pieces, and place them in a large saucepan with abundant salted water ( at least 2" of water above the potatoes).  Boil for 15-25 minutes or until the potatoes are tender (the smaller the pieces, the faster they will cook)

Step 6

Once the potatoes are done, reserve about 1 cup of the potato cooking water and set it aside

Step 7

Drain the potatoes and place them in a large bowl. Peel or squeeze out the roasted garlic cloves and add them to the potatoes along with the salt and black pepper

Step 8

While the potatoes are hot, mash them with a potato masher or a fork, add the olive oil, and as much of the reserved potato cooking water or plant-based milk needed to get a thick, creamy consistency. Add  the liquid a little bit at a time, to make sure the potatoes are not runny

Step 9

Add the capers and sundried tomatoes and mix well

Step 10

Place a clean piece of parchment paper on the baking sheet you used to bake the mushrooms. Arrange the mushrooms on the baking sheet and fill each one with a heaping pile of mashed potatoes ( the amount will depend on the size of the mushrooms). Pour the marinara sauce, top with mozzarella cheese and crumbled feta, and bake for 15 minutes