Step 1 Using a standard vegetable peeler, peel the skin from the oranges in strips. Juice the oranges, saving 1 cup; the remainder can be saved for another use.
Step 2 Place the juice, 1 cup sugar and the water into a medium saucepan and bring to a simmer over medium-high heat, then add the peel. Continue to heat until the mixture comes to a boil.
Step 3Reduce the heat to medium and continue cooking until the peel appears slightly translucent and very soft, approximately 10 minutes. Remove from heat and cool to room temperature.
Step 4While the orange peel is cooling, skin the tomatoes: Cut a small X at the base of each tomato. Bring a large pot of water to a boil, then drop the tomatoes in the water, several at a time. Remove the tomatoes with a slotted spoon when the skin has just begun peeling from the X, about 20 to 30 seconds. Immediately place the tomatoes in an ice bath to stop the cooking, then peel the skin from the tomatoes when they are cooled. Repeat until the skins are removed from all the tomatoes.
Step 5Seed and roughly chop the tomatoes and place them in a large pot. Stir in the remaining 5 cups sugar.
Step 6When the orange is cooled, chop the orange peel. Place the chopped orange peel in a small bowl, reserving the poaching liquid.
Step 7Heat the tomato and sugar mixture over high heat, stirring frequently with a large wooden spoon or heat-resistant spatula. Cook the marmalade until it begins to thicken, about 45 minutes to an hour. (It will boil and bubble furiously while it cooks; stir frequently but not constantly.)
Step 8Reduce the heat to medium-high and add the poached orange peel, salt and pepper, stirring constantly. Taste the marmalade. If you prefer stronger citrus notes, add some of the reserved poaching liquid now. Continue cooking until the marmalade is very shiny with a thick consistency.
Step 9Refrigerate the marmalade or can according to the jar manufacturer's instructions.