To prepare the marinade, break the rosemary down into smaller sprigs by pinching it off of the stem. Next, using a peeler, remove two 1" x 4" -inch pieces of zest from the lemon. Trim off any white pith. Peel and lightly crush each clove of garlic. Lightly crush the juniper berries with the side of a knife. Place everything into a bowl, along with the fennel seeds, crushed red pepper flakes, salt, pepper, white wine vinegar and white wine. Mix to combine. Set aside.
First clean the mushrooms and then cut in half.
To sear the mushrooms, preheat a large, stainless-steel pan over medium-high heat until very hot. Add the oil, immediately followed by the mushrooms. Let sit for a minute or so without moving to ensure they brown nicely. Toss a few times until nicely browned on one or two sides. Do not cook the mushrooms all the way through.
Once done, transfer the mushrooms to a plate to cool. Return the pan to the stove and turn the heat down to medium.
Add the marinade to the pan and simmer for approximately 5 minutes to infuse the flavors.
Transfer the mushrooms to a heatproof container and pour the warm marinade over top. Let cool to room temperature. Cover and refrigerate over night.
Drain the liquid from the mushrooms and discard the marinade. Place the mushrooms, along with the aromatics into a jar. Add enough olive oil to cover.
These mushrooms can be stored in the refrigerator for up to 1 month.