Preheat oven to 425°F. Line a large sheet pan with parchment paper or foil.
In a large bowl, toss the halved potatoes and sweet onion wedges with 1½ tbsp olive oil, half the garlic, half the herbs, ½ tsp salt, and ¼ tsp pepper. Spread in a single layer on the prepared sheet pan.
Roast for 20 minutes, until potatoes are beginning to turn golden.
While the potatoes roast, pat the chicken thighs dry and coat with the remaining olive oil, garlic, herbs, salt, pepper, and the juice of one lemon.
Push the potatoes and onions to the sides of the pan. Nestle the chicken thighs in the center and tuck the lemon slices and asparagus around everything.
Return to the oven and roast for 20–25 minutes, until the chicken is cooked through (internal temp of 165°F) and the asparagus is tender with slightly caramelized tips.
Finish with a squeeze of fresh lemon and a pinch of flaky sea salt. Serve straight from the pan.