Step 1

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.

Step 2

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.

Step 3

In a bowl, whisk together cream, half-and-half, 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.

Step 4

Transfer half the ice cream to a 1-quart freezer container; drizzle with half 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.