In a medium saucepan, bring 4 cups of water to a boil Add the salt and stir in the oats and cinnamon. Reduce heat to a simmer over medium low heat and cook undisturbed until the oats are tender and water has been absorbed, about 40 minutes.
To make the Berry Compote:
While the oats are simmering, place the mixed berries in another saucepan with the 3 tablespoons granulated sugar over medium heat. Cook, stirring occasionally, until the berries are tender, 8-9 minutes (a little longer for frozen).
Remove the oats from heat and stir in the milk, heavy cream, and brown sugar. Divide among serving bowls and top with the warm berry compote. Serve immediately.