Lightly oil a 9 x 13 inch cake pan, using spray oil.
In a large skillet over medium heat, add the olive oil. Once the oil starts to shimmer, add in the onion, and cook for about 5 minutes, or until soft and slightly translucent.
Add in the sliced mushrooms a little at a time as to not crowd the pan, about 2 minutes after each addition. I like to slice the mushrooms as I go, adding a good handful at a time until the mushrooms are all gone.
Add in the garlic, half the salt, pepper, and nutmeg after all the mushrooms have been added to the pan, and cook the mixture until the mushrooms have lost all their liquid, and have browned a bit on the edges, about 5 minutes.
Put the chopped beet greens on top of the mushroom mixture, and using tongs, continually toss until the beet greens are wilted and cooked down. Remove from heat, and set aside.
Heat a medium pot over medium heat, and add the water. Bring to a gentle boil, and slowly add in the chickpea flour while whisking continually.
When all the flour is fully mixed into the water, and there are no lumps, switch to a large wooden spoon, and cook for 8 minutes, until the mixture has thickened slightly.
Add in the remaining salt, nutritional yeast, scallions, and mushroom/beet greens mixture, mixing well.
Working quickly, spread the hot chickpea mixture in your 9 x 13 pan, smoothing over the top. Allow to cool for 15-20 minutes, or until cool to the touch.
Using a sharp knife, slice once long-ways in the center of the pan, then cut into thirds the other way, making 6 squares.
Heat vegetable oil in a medium skillet, and working in batches, fry the squares on each side for about 4 minutes, or until a golden brown crust forms. Remove to a paper towel lined plate.
Once the frying is finished, serve on a bed of lettuce with tomatoes, or the panisse can be used to make a hearty breakfast sandwich with fresh spinach and tomatoes, as shown.