Grate zest from oranges to equal 1 Tbsp. Repeat with grapefruit. Grate zest from lemon to equal 1 tsp.
Peel and section oranges, grapefruit, and lemon, holding fruit over a bowl to collect juices.
Stir together zest, fruit segments, sugar, kosher salt, 1/3 cup fruit juices, and 1 3/4 cups water in a large saucepan; bring to a boil. Reduce heat, and simmer, stirring occasionally, 50 minutes or until a candy thermometer registers 225 and mixture is slightly thickened. Cool completely (about 1 hour; mixture will thicken as it cools).
Pour into 2 (8-oz.) jars or airtight containers, and chill 24 hours. Store marmalade in refrigerator up to 3 weeks.
Try These Twists!
Orange Mimosa Marmalade: Prepare recipe as directed, substituting 2 Valencia or navel oranges for grapefruit and 1 3/4 cups sparkling rose or sparkling white wine for water.
Citrus-Vanilla Bean Marmalade: Split 1 vanilla bean lengthwise, and scrape out seeds. Prepare recipe as directed, stirring vanilla bean and seeds into fruit mixture with zest in Step Discard vanilla bean before pouring mixture into jars.
Ginger-Citrus Marmalade: Prepare recipe as directed, stirring in 1 Tbsp. minced fresh ginger with zest in Step
