Steam the portobellos in a covered steaming basket, over a pot filled with about 5cm of simmering water. Steam until completely soft, tender and flexible (you may need to do this in two batches), about 10 minutes. When cooked, remove and place between two tea towels. Press down on the towels to help the mushrooms release their liquid. Place the mushrooms on a cooling rack and allow them to dry and cool completely.
Take one portobello, top facing down, and place 1/4 cup of the chilled cashew cheese in the middle. Place another portobello on top. Gently press and mould the portobellos together. You can also use a big piece of cling wrap – place under the bottom portobello, gather the ends together and twist – to help mould the portobello's together. Repeat with the other portobello's.
Mix the flour and chopped rosemary together and place on a large plate, pour the almond milk into a large bowl, and place the panko on large plate. Coat the portobello's with the flour mix, then the almond milk, then the panko, making sure to cover the sides as well. Repeat this coating process one more time, then allow the coated portobello burger to rest on a piece of baking paper while you repeat this process with the other 3 burgers.
Heat your peanut oil to 190°C / 375°F use a deep fry thermometer, then gently place your burgers into the oil (fry two at a time) using a slotted spoon. Move the burgers around as they cook so that they don't sit on the bottom, and so they fry evenly. Cook until golden brown and crispy, about 4 minutes. Transfer to a tea towel lined plate, sprinkle both sides with a little rosemary salt and repeat with the remaining two burgers.