Combine the vinegar, maple syrup, and lemon juice in a small bowl. Set aside.
In a 12-inch nonstick skillet, heat the oil over medium heat. Add the Broccoflower, mushrooms, garlic, rosemary, and 1 tsp. salt; toss well to coat.
Cover the pan with the lid ajar by about 1 inch. Cook, stirring occasionally, until the florets start to brown, 8 to 10 minutes. As the mixture cooks, you should hear a gentle sizzle.
Uncover, turn the heat down to low, and cook, stirring occasionally and then pushing the vegetables back into a single layer so that most have direct contact with the pan, until the vegetables are tender and well browned, 8 to 10 minutes.
Remove the pan from the heat and immediately stir in the vinegar mixture and the butter. Stir gently until the butter has melted. Discard the rosemary sprigs and season to taste with more lemon juice and salt. Serve right away or let sit off the heat, partially covered, until ready to serve, and then reheat gently over medium-low heat.