Zest the oranges - you want about 2 tsps of zest. Add the zest, eggs, and milk to a wide shallow bowl. Whisk to break up the yolks.
Peel the oranges. Roughly chop one orange and set aside to serve with the French toast.
Add the other two to a pan along with the marmalade. Squish the segments with your hand or a masher to break them down. Cook over a medium heat until it reduces and thickens to a sauce, about 5-7 mins.
Cut up the butter into small pieces. Add a few knobs to a pan over medium heat. Soak the bread in the egg mixture, flipping a few times to make sure it’s soft without falling apart.
Once the butter starts to melt in the pan, add the bread and cook until golden on both sides. Repeat with the rest of the bread adding more butter for each batch.
Divide the French toasts between plates, pour the sauce over the top, and serve with the reserved orange segments.