Soak the spinach leaves and mustard greens in a large bowl of cold water. Let them sit a few minutes to soak which will loosen any dirt clinging to the leaves, then pull out the leaves. Finely chop the spinach and mustard greens. One way to easily chop the large mustard green leaves is to stack about 5 large leaves on top of each other. Roll them up like a cigar, then thinly slice the roll. This technique of slicing a “cigar†of stacked and rolled leaves is called “chiffonade.†At this point, you can easily chop the leaves more finely. Repeat this stacking, rolling, slicing, and chopping process with the rest of the leaves.
Make the spice blend by combining the coriander seeds, cumin seeds, black peppercorns, cloves, cardamom seeds, and dried red chili in a coffee grinder or spice grinder. If you are using a coffee grinder, you will probably want to wipe the grinder before and after with a damp paper-towel.
Prepare the paneer by cutting it into inch-size cubes.
Pan-fry the paneer in a wide skillet with about 1/4 cup or less of oil. Rotate the cubes to fry on all sides until the color is honey-brown. This will take about 7-10 minutes. Let them drain on a paper-towel.
Slice the onion. Roughly chop the garlic cloves. Slice the 2-inch piece of ginger into long slices.
Heat the 2 tablespoons of oil in a skillet over medium heat. Add the onion, garlic, and ginger, and stir-fry until the onion is light brown, 8-10 minutes. Remove the skillet from the heat, and stir in the spice mixture you made in the grinder, plus the turmeric. The heat from the onion should be just right to lightly cook the spices without burning them.
Transfer the onion and spice mixture to a blender or food processor. Add the tomato paste and 1/4 cup of the water. Puree to create a smooth paste. Return the paste to the skillet. Pour the remaining 3/4 cup water to the blender or food processor, and whir the blades to wash it out and capture the remaining paste. Add this to the skillet as well.
Place the skillet over medium heat. Pile handfuls of the greens into the skillet, cover it, and let the steam wilt them. Stir, and repeat with the remaining greens. Once they are all wilted, cover the skillet and cook, stirring occasionally, until the greens are broken down to a sauce-like consistency and are olive-green in color, 10-15 minutes.
Stir in the salt, garam masala, paneer cubes, and the optional cream. Continue simmering the curry, covered, stirring occasionally, until the cheese and cream are warmed through, 5-8 minutes.