Prepare topping: In a medium bowl, mix and squeeze crumble ingredients with your hands until well blended and clumps form. Package crumble topping, plus rhubarb and 1/2 lemon for filling, in containers or resealable plastic bags.
In a bowl, combine granulated sugar, cornstarch, orange zest, cinnamon, and salt; package in a container or resealable bag.
Chill all ingredients in a cooler.
Prep charcoal (see “Going Dutch,” below). In a 4-qt. (10-in.) camp dutch oven, combine rhubarb, juice from 1/2 lemon, and orange zest mixture. Add topping and set lid in place.
Arrange coals for baking with 8 coals under oven and 17 on lid (including 2 in center). Cook crumble 30 minutes, then add 5 more coals on top and 5 underneath. Continue to bake until crumble is bubbling in center, 5 to 15 minutes more.
Remove dutch oven from fire, uncover, and let crumble cool at least 15 minutes. Serve with whipped cream.
A dutch oven lets you simmer, sauté, and even bake with a fire’s embers. Or, for more heat control, light the precise number of charcoal briquets a recipe calls for and you can literally count your way to success.
PACK SUPPLIES. Bring fuel for a wood fire, or charcoal briquets, chimney starter, and newspaper; butane lighter; 4-qt. (10-in.) and 6-qt. (12-in.) camp dutch ovens with feet and flanged lids (lodgemfg.com); grilling tongs; grilling gloves.
PREP THE FIRE. In a fire ring with the grate pushed away, build a campfire and mound about 3 qts. of hot embers to the side. Or half-fill a chimney starter with charcoal briquets, ignite using newspaper, and let burn until spotted gray, 15 to 20 minutes; as you cook, ignite more coals as needed.
ARRANGE THE COALS. For sautéing, use tongs to clear an even layer of coals and set the pot on top. For baking or simmering, arrange 8 to 10 briquet-size coals in a circle a little smaller than the dutch oven. Set on top with lid in place; arrange 15 to 16 more coals around lip of lid and 2 coals in center.
TWEAK THE TEMP. Lift handle with tongs to check food. To decrease heat, scrape away some fuel. To increase heat or cook past 30 minutes, add 5 to 6 coals every 30 minutes beneath dutch oven and 5 to 6 on top.