Grease the inside of your slow cooker with melted butter, coconut oil, or spray for easier clean-up.
In your slow cooker, combine 2 cups steel-cut oats, 4 cups water, 3 cups milk, 1 ½ cups strawberries, 1 teaspoon cinnamon, ¼ cup maple syrup, 1 teaspoon vanilla extract, and a dash of sea salt. Stir well.
Place the lid on the slow cooker and set it to cook on low for 6 to 8 hours, depending on your preferred texture.
If you're short on time, cook on high for 3 to 4 hours.
Give the oatmeal a good stir before serving and mix in 1 cup plain Greek or regular yogurt.
Serve warm, topped with additional strawberries.
Store any leftovers in an airtight container for up to 5 days.