Step 1: Prepare the fruit. Wash and hull the strawberries, then quarter them. Thoroughly wash the oranges and, using a sharp knife or mandoline, thinly slice them, including the peel and pith. Remove any seeds. If preferred, you can zest the oranges and then segment them, finely chopping the zest separately.
Step 2: Combine ingredients. In a large, heavy-bottomed pot or Dutch oven, combine the prepared strawberries, sliced oranges (or chopped zest and segments), and granulated sugar. Stir gently to mix everything thoroughly.
Step 3: Macerate the fruit. Let the fruit and sugar mixture sit at room temperature for 30 minutes to 1 hour. This allows the fruit to release its natural juices, aiding in sugar dissolution and pectin extraction.
Step 4: Cook the marmalade. Place the pot over medium-low heat. Stir constantly until the sugar is completely dissolved. Once dissolved, increase the heat to medium-high and bring the mixture to a rolling boil that cannot be stirred down.
Step 5: Simmer and thicken. Reduce the heat to medium-low and continue to simmer, stirring occasionally to prevent sticking. Cook until the marmalade thickens and reaches its gelling point. This can take 30 minutes to over an hour.
Step 6: Test for set. To check for doneness, perform the cold plate test: Spoon a small amount onto a chilled plate from the freezer. Let it cool for 1 minute, then push it with your finger. If it wrinkles, it's ready. You can also use a candy thermometer; the gelling point is typically around 220°F (104°C).
Step 7: Jar the marmalade. While the marmalade is cooking, sterilize your canning jars and lids. Carefully ladle the hot marmalade into the sterilized jars, leaving 1/4 inch headspace. Remove any air bubbles, wipe the rims clean, and affix the lids and bands finger-tight.
Step 8: Process and store (optional but recommended). For shelf-stable storage, process the filled jars in a boiling water bath for 10 minutes (adjust for altitude). Remove jars and let cool undisturbed for 24 hours. Check seals; unsealed jars should be refrigerated.