Step 1 For the onion jam, heat the oil in a saucepan over medium heat. Cook onion, stirring occasionally, for 20 minutes or until lightly caramelised. Add the sugar and vinegar and cook, stirring occasionally, for 5 minutes or until caramelised. Season with salt. Remove from the heat and set aside to cool.
Step 2 For the hollandaise, whisk the yolks, egg, mustard and lemon juice in a heatproof bowl until combined. Place bowl over a saucepan of just-simmering water (don’t let the bowl touch the water). Whisk vigorously for 5 minutes or until the mixture is thick and pale. Slowly add the butter, a little at a time, whisking constantly, until the sauce thickens. Add the chilli powder and season with salt.
Step 3 To serve, spread the muffin bases with the onion jam. Top with the mortadella and poached eggs. Spoon over the hollandaise sauce. Drizzle with chilli oil and sprinkle with chives. Sandwich together with the muffin tops.