Add the egg, lemon juice, white vinegar, mustard seeds and half the sunflower oil to a bowl - in that order. Make sure to add the egg first as you want the egg to be sitting at the bottom of the bowl with the oils on top.*
Using an electric mixer/ whisker or a hand blender, beat until well combined. I used an immersion blender and probably blended for around 30-40 seconds. While blending I kept the blender touching the bottom of the bowl at all times and you'll notice how the mayonnaise starts to form near the source of the blades.
Keep mixing while gradually adding the rest of the oil until you achieve a thick and creamy consistency. The key here is when adding the oil, to really take this part gradually or you'll end up with a soupy mess. Adding practically drops at a time/ in a thin stream, gradually, is the key to a properly emulsified homemade mayonnaise (check how I did it in the video).**
Store in a glass air-tight container in the fridge for 5-7 days. The general rule of thumb is that it will last as long as your eggs would have, especially as the vinegar and lemon help to act as preservatives. However, I tend to keep mine for a week maximum.
* It's good to then give the ingredients a moment to 'settle'. This will reduce the risk of the mayonnaise failing when blending, is they're all in their correct' layers'. If you're wanting to make this recipe even simpler, then you can actually blend it all straight into the jar you'll be keeping it in by using the above method and making sure the ingredients are in their layers before blending. My immersion blender is a bit too big for the jar, unfortunately. ** At this point taste your mayonnaise and decide if you want to add any additional salt or lemon juice for added flavour.Note* You can use olive oil for this recipe. However, if you do then I suggest using a light olive oil to not overpower the mayonnaise too much.Read the blog post for more tips and how to fix 'broken' mayonnaise.