In a medium bowl or mason jar, combine the rolled oats, chia seeds, cinnamon, and salt. Whisk these dry ingredients together to evenly distribute the spices—this prevents clumping and ensures consistent flavor throughout. In a separate small bowl, whisk together the Greek yogurt, almond milk, maple syrup, and vanilla extract until smooth and well combined. I find that whisking the wet ingredients separately helps me avoid lumpy yogurt that can stick to the bottom of the bowl.

Pour the wet ingredient mixture from Step 1 into the bowl with the dry ingredients and stir well until everything is combined. Add the frozen blueberries and gently fold them in—using frozen berries straight from the freezer means they'll slowly thaw overnight and won't break down into mush. Cover the bowl or jar with a lid or plastic wrap and refrigerate for at least 8 hours or overnight. I like to prepare my overnight oats in the evening so they're ready for grab-and-go breakfast, but you can also make them up to 2 days ahead.

Remove the overnight oats from the refrigerator and give them a good stir, adding a splash more almond milk if they've thickened too much overnight—the consistency should be creamy and spoonable, not dry. Transfer to a serving bowl or eat straight from the jar, then top with the chopped roasted walnuts for crunch and extra nutrition. Serve chilled and enjoy immediately.