Juniper berries add an earthy, pine-tinged flavor to this make-ahead relish; it's a perfect complement to rich meats and cheeses. Purchase them at specialty and spice stores or online.
In a large skillet, heat 1 tablespoon of the oil over medium-high heat. Carefully add onions (they will spatter); cook and stir for 2 minutes or until browned. Using a slotted spoon, transfer to a heatproof bowl. Add half the mushrooms to the skillet; cook and stir for 3 minutes or until mushrooms begin to brown but are not fully cooked. Transfer to bowl with onions. Add remaining oil to skillet; cook remaining mushrooms and transfer to bowl.
Add garlic to skillet; cook and stir for 1 minute. Add wine, vinegar, thyme, orange peel, bay leaves, juniper berries, salt and crushed red pepper to pan. Bring to a boil, reduce heat and simmer for 4 minutes. Remove from heat; stir in orange juice. Pour over onions and mushrooms. Cool to room temperature, then chill, covered, for at least 24 hours or up to 1 week, stirring occasionally during the first 24 hours. To serve, transfer to a bowl with a slotted spoon; drizzle with olive oil, if desired.