Heat oven to 375 degrees. Cut rhubarb into 1/4-inch-thick cubes, yielding about 3 cups. Tear brioche into 1/2-inch pieces, yielding 3 to 3 1/2 cups.
Butter a shallow 1-quart baking dish. In medium bowl, combine melted butter and brioche; cover bottom of dish with 1 to 1 1/2 cups brioche. In another bowl, combine rhubarb, apple, brown sugar, cinnamon, nutmeg, and lemon juice. Grate zest; add to mixture. Let sit until juices begin to run, about 5 minutes. Spread half of rhubarb
mixture over brioche. Sprinkle with 1 cup brioche. Add remaining rhubarb mixture and juices; cover with remaining 1 cup brioche. Dot with 1 tablespoon butter. Spoon 6 tablespoons warm water over top.
Cover; bake 25 to 35 minutes on the middle rack, until rhubarb is tender. Increase heat to 400 degrees; uncover, and bake 10 to 15 minutes more, until rhubarb is soft, top is crusty, and juices begin to bubble at edges of baking dish. Serve warm with vanilla ice cream.