Heat oven to 200 degrees F. Mix popped corn and cashews in roasting pan.
In 3-quart saucepan, combine sugar, syrup, butter, salt, cream of tartar. Bring to boiling, stirring, over medium-high. Without stirring, boil until hard-ball stage, 260 degrees F on candy thermometer, 6 to 8 minutes. Remove from heat; quickly stir in baking soda. Pour over popcorn in roasting pan; stir to coat.
Bake in 200 degree oven 1 hour, stirring occasionally. Spread popcorn on waxed paper-lined baking sheets. Let cool. Break popcorn apart. Store airtight at room temperature 1 week. Makes 20 servings (1 cup each)
