Soften the onion and carrots in the oil over medium heat, about 8 minutes, until the onion turns translucent and the carrot edges give.
Add the garlic for 30 seconds, then the chicken thighs, broth and salt.
Bring to a bare simmer, never a boil, and cook covered 30 minutes until the thighs shred at a fork.
{'Blend in batches while hot': "fill the jar no more than halfway, remove the lid's center cap, and cover the gap with a folded towel. Blend each batch a full 90 seconds past when it looks smooth."}
Pass the blended soup through a fine mesh strainer, pushing with a spatula, for the silkiest result.
Taste and season now, not before. Blending mutes salt, so the pot needs its final adjustment here.