Wash mushroom caps well, carefully removing any dirt. Pat dry.
Throw all marinade ingredients together in a large plastic bag, mash around making sure it is well combined. Place mushrooms in the bag, seal it up, roll contents around to coat, and let sit in the fridge for up to 12 hours (minimum 1 hour). If you think of it, take the bag out from time to time and roll it around – this ensures that the mushrooms are well coated with marinade.
Prepare the grill by heating it up to medium-high. Place mushrooms on lightly oiled grill, gill side down, close grill lid and cook for 5-7 minutes or so, until grill marks appear on the underside. Flip, and repeat gill side up. (Doing it in this order allows the juices to accumulate in the middle) The mushrooms are cooked when they are warm through and have lost a little of their “plumpness”.
Meanwhile, in a food processor, puree the clove of garlic, 1 tablespoon thyme, the white beans, and 1 teaspoon oil until smooth. If mixture is too thick to spread, add a bit of water. Season with pepper, and pulse to combine.
To serve, top grilled mushroom with the white bean spread and sprinkle with corn.