Heat canola oil in a large cast iron skillet over medium heat. Working in batches, add chicken to the skillet in a single layer and cook until golden brown, about 4-5 minutes. Reduce heat to medium low. Using tongs, flip and cook, covered, until cooked through, reaching an internal temperature of 165 degrees F, an additional 8 minutes; set aside and keep warm.
Melt 1 tablespoon butter in the skillet over medium low heat. Add garlic, shallot and rosemary, and cook, stirring frequently, until fragrant, about 2 minutes; season with salt and pepper, to taste.
Whisk in flour until lightly browned, about 1 minute.
Stir in chicken stock, scraping any browned bits from the bottom of the skillet. Bring to a boil; reduce heat and simmer, stirring occasionally, until reduced and thickened, about 4-5 minutes.
Stir in lemon juice and remaining 1 tablespoon butter; season with salt and pepper, to taste. Return chicken to the skillet.