Position a rack in the middle of the oven and preheat to 375°F. Lightly grease an 8x8-inch baking dish and, if you like easier removal, line it with a strip of parchment.
Prepare the peaches: reserve about 1 1/4 cups of peaches thinly sliced for the topping. Chop the remaining peaches into bite-sized pieces and place them in a medium bowl; toss the chopped peaches with half the maple syrup (about 2 tbsp) and the lemon zest so they macerate while you mix the oat base.
In a large mixing bowl whisk together the almond milk, remaining maple syrup, melted coconut oil (or butter), vanilla and almond extract until smooth.
Add the rolled oats, cinnamon and sea salt to the wet mixture and stir until evenly combined. Fold in the macerated chopped peaches so they’re distributed through the oatmeal batter.
Pour the oat mixture into the prepared baking dish and smooth the top. Arrange the reserved thin peach slices across the surface in overlapping rows or a fan pattern for an attractive finish.
Bake uncovered for about 40–45 minutes, or until the center is set and the top is lightly golden and fragrant. If the edges brown too quickly, tent loosely with foil for the last 10 minutes.
Remove from the oven and let the baked oatmeal rest at room temperature for 10 minutes before slicing. Serve warm with a splash of almond milk or yogurt, if desired.
To store: cool completely, cover tightly and refrigerate for up to 4 days. To reheat a whole pan, cover with foil and warm at 350°F for about 20 minutes. For single servings, microwave for 60–90 seconds or bake in a toaster oven at 350°F for 5–10 minutes.