Preheat the oven to 375ºF. In a small bowl, stir together the Dijon mustard, maple syrup, olive oil, and soy sauce. Mince the garlic and chop the rosemary. Stir both into the marinade.
Arrange the chicken thighs in the dish so that they are close, but not overlapping. Pour the marinade on top and then spread it around until the thighs are completely covered.
Bake the chicken thighs in the preheated oven for 45-50 minutes, or until they are cooked through and the liquid around the thighs is simmering. Spoon some of the liquid over the thighs, return them to the oven, then switch the oven's setting from bake to broil. Broil for about 5 minutes, or until the top of the chicken is golden brown. Watch the chicken closely as it broils, as broilers can vary.
Use a spoon to drizzle the cooking juices over top of the chicken after baking. Avoid using a brush as this will wipe off the baked on marinade.