Preheat the oven to 375 degrees F. Put the poblanos directly on top of the burner on high heat, turning with tongs until completely charred black, about 5 minutes. Place in a bowl and cover with a dish towel for 10 minutes. Rub off the skin with your hands or a paper towel, remove the seeds and cut into strips. Reserve.
Heat the vegetable oil in a large skillet over medium-high heat. Add in the onions, sprinkle with salt and saute until caramelized, stirring often, 25 to 30 minutes. Reserve.
Meanwhile, halve the fingerling potatoes and toss in a bowl with olive oil, half of the thyme, the rosemary and half of the garlic, and sprinkle with salt and pepper. Roast on a baking sheet at until golden brown, 25 to 30 minutes.
Toss the grape tomatoes with olive oil, salt, the sugar, the remaining 2 sprigs thyme and half head garlic and drizzle with balsamic vinegar. Roast on a baking sheet until softened and beginning to char, 15 to 20 minutes.
For the queso fundido:
Increase the oven to 400 degrees F. Melt the butter in a saucepot over medium heat, and then stir in the flour and cook, 2 minutes. Add the milk and cook until thickened, whisking occasionally, 8 minutes. Turn off the heat, add the cheeses and combine well. The mixture will be dough-like; it won't get oozy until it gets in the oven.
Transfer the mixture to a 10-inch cast-iron skillet or shallow baking dish, and spread evenly. Bake 6 minutes, stir and bake another 6 minutes, then stir again. Put under the broiler until the cheese is browned in spots. Serve immediately with the dipping sides of the roasted poblanos, caramelized onions, roasted fingerlings and tomatoes, gherkins, slices of baguette and dried sausage.
If you don't have a gas burner, you can char your poblanos under the broiler as well.