Preheat the oven to 425°F (220°C).
In a small bowl, whisk together the olive oil, balsamic vinegar, honey, grated garlic, and dried thyme or oregano.
Place the chicken in a large bowl and season with salt and black pepper.
Pour the dressing over the chicken and toss well to coat.
Transfer the chicken and all of the marinade to a large sheet pan. Add the sliced onion around the chicken.
Bake on the center rack for 25 to 30 minutes.
Remove the pan from the oven and add the asparagus. Drizzle the asparagus with a little olive oil and season lightly with salt and pepper.
Return the pan to the oven and bake for another 20 to 25 minutes, or until the chicken is cooked through and the asparagus is tender.
Top with crumbled feta before serving.
Serve this Balsamic Chicken Sheet Pan Meal warm.