Prep the cabbage by coring and slicing it into thin shreds; slice the onion, if using.
Melt the butter in a large skillet over medium heat.
Bloom the caraway seeds in the butter for 30 seconds until fragrant.
Stir in the onion and cook, stirring, until soft and translucent, 2 to 3 minutes.
Add the cabbage, salt, and pepper. Toss to coat with the butter.
Pour in the water, cover, and steam until the cabbage softens, about 5 minutes.
Uncover and cook, stirring often, until tender but still bright, 4 to 6 minutes.
Finish with lemon juice and parsley. Taste and adjust salt and pepper, then serve warm.