Place rack in the upper third of the oven and preheat oven to 375 degrees F. Line a large baking sheet with parchment paper or a Silpat mat. Place the plums cut-side up to one side of the baking sheet. Drizzle with olive oil and honey, then sprinkle with cinnamon.
Place the old fashioned rolled oats, walnuts, coconut, cinnamon, and salt in a medium bowl. Drizzle with honey and olive oil, then toss until evenly moistened. Spread in a single layer on the open side of the baking sheet beside the plums.
Place the baking sheet in the oven and roast for 12-15 minutes, until the plums begin to bubble and the granola is lightly golden and fragrant.
Meanwhile, prepare the steel cut oats. In a large saucepan, bring the water and salt to a boil. Add the steel cut oats. Reduce heat, cover, and simmer for 5 to 7 minutes, cooking to the desired thickness and chewiness and stirring occasionally. Remove from heat and let stand, covered, for 2 minutes.
To serve, spoon 1/4 of the steel cut oats into each bowl. Top with milk (if desired), roasted plums, a sprinkle of granola, and an extra drizzle of honey. Enjoy immediately.