Remove the casing from each sausage and shape into 1 cm thick patties.
Heat the oil in a medium frying pan over a medium-high heat. Fry the sausage patties until they are golden on both sides, around 8-9 minutes. Set aside on a plate.
Preheat the oven to 175℃/155℃ fan (350℉/310℉).
Crack the eggs into a bowl and whisk together until slightly frothy. Add in the milk and season with salt and pepper. Whisk until combined.
Melt the butter in a medium non-stick frying pan over a medium-low heat.
Add in the eggs and leave undisturbed until just set, 1-2 minutes. Use a rubber spatula to push the eggs into the centre and then spread them out again. Leave for another few seconds until set again. Repeat until the eggs are almost cooked, but still slightly wet on top. Transfer to a bowl and set aside.
Slice the croissants in half. Spread the bottom half of each with 1 tsp of mustard.
Place 2 sausage patties onto each croissant. Top each with ¼ of the scrambled egg, a sprinkle of chives, and ¼ cup of cheese. Place the top half of each croissant on top of each sandwich.
Place the croissants on a lined baking tray and bake until the cheese has melted, around 8-10 minutes. Enjoy!
