Process the diced cold butter, GF oats, almond flour, brown sugar, granulated sugar, salt, and cinnamon. If you don’t have a food processor, place them in a medium bowl and toss them with your fingertips to combine. Let rest in the freezer.
In a large bowl place the sliced plums, corn starch, and lemon juice, and mix.
Place the plum mixture into the desired molds. I used individual ramekins, but you can also use a baking dish, 8” square pan, or a 9-in. pie plate. Let rest for 10 minutes.
Sprinkle the oat mixture over the plums and the chopped toasted almonds (optional).
Bake for about 40 minutes, until the topping is golden brown and the plums are tender and bubbling.
Serve warm with whipped cream or a scoop of ice cream if desired.
Notes
Storage: keep leftovers in the refrigerator for 3 days, covered in plastic wrap or an airtight container. I don't recommend freezing it.