Heat olive oil in a large sauté pan over medium-high heat. Add the mushrooms and give them a toss in the olive oil. Cook the mushrooms until they become brown on the outside and tender, stirring occasionally, about 5 minutes. Sprinkle with salt and pepper at the end. Transfer to a bowl and set aside.Turn the heat down to low and butter to the same sauté pan you cooked the mushrooms in. Add the sliced onions and toss to coat in the butter. Continue to cook, stirring often, until the mushrooms turn an amber brown, about 30 minutes. Season with salt and pepper to taste.While the onions caramelize, slice the baguette into 1/2-inch thick rounds. Preheat an outdoor grill or indoor grill pan. Grill the bread on each side for 30 seconds to 1 minute, or until golden brown on each side.Add the mushrooms to the pan with the caramelized onions; toss to combine. Stir in the balsamic vinegar and honey. Continue to cook for 3 minutes. Season the mixture with salt and pepper to taste.Top each piece of toasted bread with a small mound of the onion mixture. Top with a sprinkle of blue cheese crumbles and pinch of fresh thyme. Serve immediately.