Remove the central stems from the collards, then chop the leaves. Combine the chopped collards, spinach, garlic, and onion in a steamer (or place in a heavy-bottomed pot with a lid). Season with salt, red chili powder, and turmeric. Add half a cup of water and close the lid. Steam on medium-high for 10 minutes, reduce heat to low for another 15 minutes, then turn off the heat and check to ensure that the greens have wilted and turned a dark green color. Onions should have melted and garlic cloves should be soft and mushy. Add the flour and start stirring immediately with a large, sturdy spoon. Keep stirring to mix in all the flour and continue until the leaves disintegrate and come together as a mash. This should take approximately 10 minutes. In a separate pan over high, heat 1 tablespoon of the ghee until shimmering. Tip the pan so as to pool the hot ghee into one side of the pan. Add the mustard seeds, cumin seeds, and coriander seeds, let them splutter, then add ginger. Once it is fragrant add a pinch of red chili powder. Pour tadka over the saag and mix in. Transfer the saag to a serving bowl and garnish with an additional tablespoon of ghee.