3 cups diced fresh rhubarb or sliced frozen rhubarb
2/3 cup sugar plus 3/4 cup sugar, divided
2/3 cup water
1 tablespoon butter
1/3 cup quick-cooking oats
1 tablespoon brown sugar
1/4 teaspoon ground cinnamon
1-1/2 cups heavy whipping cream
1-1/2 cups half-and-half cream
1 teaspoon vanilla extract
Place rhubarb, 2/3 cup sugar and water in a small saucepan; bring to a boil, stirring occasionally. Reduce heat; simmer, until rhubarb is softened and mixture is thickened, 8-10 minutes. Cool completely.
In a small skillet, melt butter over medium-high heat. Add oats, brown sugar and cinnamon; cook and stir until oats are coated, 45-60 seconds. Remove to a bowl; cool completely.
In a bowl, whisk together both creams, vanilla and remaining sugar until sugar is completely dissolved. Fill cylinder of an ice cream maker no more than two-thirds full. Freeze according to manufacturer’s directions. During the last 5 minutes of processing, add oat mixture, breaking up any large crumbles before adding.
Transfer half of the ice cream to a 1-quart freezer container; drizzle with half of the rhubarb mixture. Using a knife, cut through mixtures in a zigzag pattern to swirl. Repeat with remaining ice cream and rhubarb. Freeze, covered, until firm, about 4 hours.