In a measuring cup, combine the grapeseed oil and olive oil.
In a large bowl, whisk together the egg yolks and salt for 1 minute. Add the oil a few drops at a time, whisking constantly (adding the oil too fast will prevent the mayonnaise from emulsifying and becoming thick and creamy). Once the mixture is thick and begins to resemble mayonnaise, add the remaining oil in a very slow stream, whisking constantly until the mixture is fully combined and emulsified.
Whisk in the lemon juice and hot water. Serve right away or transfer to an airtight container and refrigerate for up to 3 days.