Ramsey is catering to a social media demographic. He's much more business minded about his culinary life than Marco Pierre White is, who is business minded in his own right (he owns a couple of restaurants in the UK if I'm not mistaken) but he isn't trying to be the face of pop food culture (not anymore, at least).
The mushroom filling is just mushrooms put in a food processor until its a paste. Some people stop just before its paste like, but in this case Marco goes all the way to a fine paste. Afterwards you just cook it until all the water seems to have been removed and it no longer sizzles. He doesn't season it with anything beyond salt, but I've seen others add some "green" spices (parsley, oregano, thyme, marjoram, . . . ). Its called a Duxelle, there are many different recopies you can find online, but in this case its just mushroom paste and salt cooked until all excess water is gone.
Micro greens look better for presentation, especially on tv because it gives dishes height and more variation of color. He's been doing that for a long time now.
