In a large resealable plastic bag, add 1/2 of the minced garlic, 1 tablespoon of the chopped rosemary, 1 tablespoon of the chopped thyme, the balsamic vinegar, 3/4 cup of the strawberry jam, the Dijon mustard, 1 teaspoon of the salt, 1 teaspoon of the black pepper, and the chicken thighs.
Tightly seal the bag and massage to coat the chicken thighs with the marinade.
Transfer the bag to a large plate and refrigerate, for at least 1 hour and up to 4 hours.
About 20 minutes before the chicken thighs are finished marinating, adjust an oven rack to the middle position. Preheat the oven to 400 degrees F.
In a large bowl, add the remaining garlic, the remaining 1 tablespoon of chopped rosemary, the remaining 1 tablespoon of chopped thyme, the remaining salt, and the remaining black pepper and stir to combine.
Add the potatoes to the herb mixture.
Add 1 tablespoon of the olive oil to the herb mixture and toss to coat the red potatoes pieces. Set it aside.
Place the marinated chicken thighs on a large plate and pat them dry with paper towels. Discard the marinade.
In a large cast-iron skillet over medium-high heat, heat the remaining olive oil until it is shimmering.
Add 3 of the marinaded chicken thighs, skin-side down, to the heated olive oil and weigh them down with the bottom of a clean skillet, heavy plate, or steak weight.
Cook the chicken thighs until their skin is deep golden-brown, about 2-4 minutes.
Place the browned chicken thighs onto a clean plate, skin-side up. Repeat the browning process with the remaining chicken thighs.
In the same skillet over medium heat, add the red onion and cook, stirring frequently, until just softened, about 3-5 minutes.
Transfer the cooked onions onto the same plate as the chicken thighs.
In the same skillet, add the potatoes, in a single layer and with the cut-sides down, and cook until the potato bottoms are browned and release easily from the bottom of the pan, about 6-10 minutes; they will not be fully cooked.
Turn off the heat. Add the cooked red onions to the potatoes and toss to combine. Arrange the mixture in an even layer.
Place the chicken thighs, skin-side up, on top of the potato mixture.
In a small microwave-safe bowl, add the remaining strawberry jam and heat in the microwave until it has slightly loosened, about 10-20 seconds.
Use a pastry brush to coat the chicken thighs with the melted strawberry jam.
Sprinkle the chicken thighs with some of the extra rosemary sprigs and some of the extra thyme sprigs.
Bake the chicken thigh mixture until the potatoes are fork-tender and the meat reaches an internal temperature of 165 degrees F, about 40-45 minutes. If the chicken skin begins to darken too quickly, cover the skillet with foil.
Let the chicken thigh mixture rest for 5 minutes.
Serve with more of the extra rosemary sprigs and more of the extra thyme sprigs.