Preheat grill to medium high heat.
In a small saucepan mix all Miso glaze ingredients together. Bring to a boil over medium high heat and whisk so that sugar dissolves and a smooth paste is formed.
Brush liberally over the portobello mushrooms and set aside until ready to grill.
Toast sesame seeds in a pan for 1-2 minutes over medium high heat until golden brown and fragrant.
Using a mandolin or veggie peeler cut cucumber into long thin ribbons. Place in a medium bowl and mix with rest of ingredients and set aside.
Mix all ingredients for carrot salad in a medium mixing bowl and set aside.
Mix hoisin and mayo together in a bowl until well combined.
Grill portobellos over medium high heat top side down for 4-5 minutes until juicy and tender. Flip and grill for another minute or two.
Grill buns for a minute a side.
To assemble place hoisin mayo on top and bottom bun, place portobello then add carrot and cucumber salad. Top with bun lid. Sprinkle with toasted sesame seeds.