In a large skillet set over medium-high heat add the olive oil and the mustard seeds and cook the mustard seeds until they begin to pop.
Add the diced onions to the skillet and cook for about 4 to 5 minutes until the onions are translucent.
Add the Garam masala spice mixture, ground ginger, ground tumeric, ground cumin, chili powder, and red pepper flakes to the skillet and cook for a 2 to 3 minutes while stirring to really get the spices warmed up and activated.
Turn the heat off and add the potatoes and diced tomatoes to the skillet and toss them in the spice/onion mixture being sure to coat the potatoes well in the mixture.
Add the salt and pepper and give it one last stir.
Pour the potatoes from the skillet into a 6 quart or larger slow cooker and cover and cook for 4 to 6 hours on LOW.
When potatoes are done cooking sprinkle with cilantro or flat leafed parsley if desired for a nice pop of color and fresh flavor.