In a large pot filled with water, add 1/4 cup of vinegar, 1 tablespoon of salt, and the mushrooms. Bring to a boil and cook for 15 minutes. Drain the mushrooms and set the aside.
Gather the marinade ingredients. Dice the onion and bell pepper into small cubes. Mince the garlic.
In a saucepan, prepare the marinade by adding the water, salt, sugar, bay leaves, and peppercorns. Bring to a boil. Once the sugar and salt has dissolved, add the vinegar, bell pepper, and onion. Remove from heat and set aside.
Place diced garlic at the bottom of sealable canning jars. Fill the jars with cooked mushrooms and fresh dill. Pour the marinade in until the mushrooms are completely submerged.
Place the sealed jars in the refrigerator overnight to let the mushrooms marinate. Serve cold as an appetizer or side dish.