Preheat the oven to 400°F. Wash and dry the fresh produce. Cut the mushrooms into bite-sized pieces. Halve, peel, and medium dice the onion(s). Grate the gouda on the large side of a box grater.

In a medium nonstick pan (or large for 4 servings), heat a drizzle of olive oil on medium-high until hot. Add the mushroom pieces in an even layer. Cook, without stirring, 3 to 4 minutes, or until browned. Add the diced onion(s); season with salt and pepper. Cook, stirring occasionally, 3 to 4 minutes, or until softened. Add half the vinegar (carefully, as the liquid may splatter). Cook, stirring constantly, 30 seconds to 1 minute, or until the vegetables are coated. Turn off the heat.

Crack the eggs into a large bowl. Add 2 tablespoons of water (or 4 tablespoons for 4 servings); beat until smooth. Add the cooked vegetables and half the grated gouda; season with salt and pepper. Stir to combine. Place the pie crusts on a sheet pan, leaving them in their tins. Evenly divide the filling between the crusts. Top with the remaining grated gouda; season with salt and pepper.

Bake the quiches 19 to 21 minutes, or until the crusts are lightly browned and the filling is set and cooked through. Remove from the oven and let stand at least 2 minutes before serving.

Meanwhile, cut off and discard the stem ends of the brussels sprouts; halve lengthwise. Line a separate sheet pan with foil. Place the halved brussels sprouts on the foil. Drizzle with olive oil and season with salt and pepper; toss to coat. Arrange in an even layer. Roast 14 to 18 minutes, or until lightly browned and tender when pierced with a fork. Remove from the oven. In a large bowl, whisk together the honey and remaining vinegar; season with salt and pepper. Add the roasted brussels sprouts; season with salt and pepper. Toss to coat. Taste, then season with salt and pepper if desired. Serve the baked quiches with the finished brussels sprouts on the side. Enjoy!