Add oil to the pan and heat over medium heat. Add onion and cook, stirring often, until soft but not browned, reducing the heat if necessary, about 4 minutes. Stir in thyme (or savory) sprigs, salt and pepper. Increase heat to medium-high, add the Brussels sprouts, and cook, tossing or stirring occasionally, until tender and warmed through, about 3 minutes. Remove the herb sprigs. Add the bacon, thyme (or savory) leaves and lemon juice, if using, and toss.