To make the sauce, heat frying pan and melt butter. Fry miso paste so it can absorb the butter. Pour in milk and dissolve miso paste on a medium heat. Then leave it to simmer.
Slice the chicken. Heat frying pan on high heat. Stir-fry meat. Season with tarragon, salt and mirin.
Slice avocado. Cut bagels into half and toast.
To thicken miso sauce, dissolve cornstarch in a tiny bit of water, then pour it into the sauce and stir to thicken it. Now assemble sandwich.