Preheat oven to 300° with an oven wrack in the middle position.
Bring a kettleful (or medium saucepan) of water to a boil, then set aside.
Combine 1 ½ cups of cream with the sugar in a medium saucepan. Add in the finely grated/zested peel of the Seville oranges into the pot—zesting them straight over the pot—and stir to combine. Place over medium heat, stirring occasionally, and bringing to a gentle bubble to dissolve the sugar—about 5 minutes.
Once the cream has come to a bubble and sugar has dissolved pour in the remaining 1 ½ cups of heavy cream. Set aside to steep for about 10 minutes.
Beat the egg yolks in a separate bowl with a whisk until just combined and smooth. Once the cream has steeped pour about a generous ½ cup of the cream into the yolks, and whisk to combine. Do this once more before pouring in all the remaining cream along with 3 tablespoons of juice from the Seville oranges. Pour this through a fine mesh sieve into a large liquid measuring cup (1 to 1 ½ quarts).
Lay a clean kitchen towel onto a roasting tray—it helps to keep the ramekins from sliding around once you pour in the water—and set six 4 to 5-ounce ramekins on it (mine measure about 5” diameter, 1” deep). Carefully pour the crème into each dish. Pull the oven wrack out, set the pan on it and, even more carefully still, pour the hot/recently boiled water into the roasting pan until it comes up about halfway up the sides of the ramekins, taking care not to splash any into the crème.
Bake for about 25 – 30 minutes, until the crème has set but is still soft, with a slight wobble in the center (or read about 170° on an instant-read thermometer)
Remove from the oven and set on a wire wrack to cool completely—about 2 hours. Move them to the fridge for at least 6 hours, or up to 3 days.
Just before you’re ready to serve sprinkle each one with a t-spoon or so of the turbinado sugar and brulee them with a kitchen torch, making circular motions about an inch or two away from the surface. Serve immediately.
Seville oranges are sour oranges from Seville, Spain. If you find other oranges marked as "Sour oranges" in your grocery store or market, those will do the trick!
You can certainly make Créme Brûlée with regular oranges but it will be NOTHING like what it would be with sour oranges—there is simply no substitution for them (a combo of orange and lime gets you close with the juice, but the zest is irreplaceable).