Bring a large pot of water to a boil.

Wash and cut brussels sprouts in half if they are very small, otherwise cut into quarters.

Cook the sprouts until barely tender, 3 to 5 minutes. Drain.

Meanwhile, cook bacon in a large heavy skillet over medium heat, stirring, until brown but not crisp, 3 to 6 minutes.

Remove with a slotted spoon to drain on a paper towel.

Pour out all but about 1 tablespoon bacon fat from the pan.

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 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.

Add the bacon, thyme leaves and lemon juice, if using, and toss.