Add the pepper and salt to the ground beef and mix with your hands, then form 4 patties. (Be careful not to overmix or the texture will suffer.)Heat a large cast iron skillet over medium high heat and add the burgers to the pan. Cook until they've just about reached desired doneness, flipping halfway through cooking time. (About 4-6 minutes per side.)Drop 1 ounce cream cheese on top of each burger and cover. Turn off the burner and let the residual heat soften the cream cheese.Place 1 burger on the bottom of each bun and dollop 1 Tablespoon of jam on each. Divide the spinach evenly, then place the top of the bun on the burgers.Serve immediately.