Step 1: Prepare your canning jars and lids by sterilizing them. Wash jars thoroughly in hot soapy water or run through a dishwasher cycle. Keep them hot until ready to fill to prevent thermal shock. Place lids in simmering water.
Step 2: Wash and zest the oranges, being careful to only get the orange part, not the bitter white pith. Juice the oranges. Hull and roughly chop the strawberries.
Step 3: In a large, heavy-bottomed pot or Dutch oven, combine the chopped strawberries, orange zest, orange juice, and lemon juice (if using). Bring to a simmer over medium heat and cook for 10-15 minutes, until the strawberries begin to break down.
Step 4: In a small bowl, whisk together the pectin with 1/4 cup of the granulated sugar. This helps prevent clumping. Add this pectin-sugar mixture to the fruit in the pot, stirring constantly until dissolved. Bring the mixture to a rolling boil over high heat, stirring frequently.
Step 5: Once at a rolling boil that cannot be stirred down, add the remaining granulated sugar all at once. Stir until the sugar is fully dissolved. Return the mixture to a full rolling boil and boil hard for exactly 1 minute, stirring constantly. Watch for scorching.
Step 6: Remove the pot from the heat. Skim off any foam from the surface with a spoon. Carefully ladle the hot strawberry orange marmalade into the hot sterilized jars, leaving 1/4 inch headspace. Wipe jar rims clean.
Step 7: Place lids on jars and screw bands on until fingertip-tight. Process jars in a boiling water canner for 10 minutes (adjust for altitude if necessary). Carefully remove jars and let them cool undisturbed on a towel-lined counter for 12-24 hours.
Step 8: Check seals. Lids should be concave and not flex when pressed. Any unsealed jars can be refrigerated and used within 3 weeks. Store sealed jars in a cool, dark place for up to 1 year.