For pressure cooker, replace simmering time with 15 minutes at high pressure, then quick release.
For slow cooker, brown beef with onion in a skillet, then place it in the slow cooker with everything but the tomatoes. Cover and cook on Low for 6 hours, or until vegetables and barley are tender. Stir in the tomatoes and serve.
You can find bags of barley at most grocery stores near the dry beans and rice. Pearl barley is most commonly available, and it cooks faster than regular barley.