Bring a large pot of water to a boil. If sprout are very small, cut in half, otherwise cut into quarters. Cook the sprouts until barely tender, 3-4 minutes, then drain.
Meanwhile, cook bacon over medium heat, stirring, until brown. Place on the paper towel to get excess fat off.
Pour out bacon fat from the pan except about 1 tablespoon.
Add onion to the pan and cook for about 4 minutes stirring often. Stir in thyme and any fresh herbs, salt and pepper.
Increase heat to medium-high, add the brussels sprouts and cook, tossing and stirring occasionally, until tender and warmed through, it takes about 3 minutes. Add bacon and lemon juice, if using, and toss.