Split the chilies by either tearing the tops off, or using kitchen scissors to cut them open. Remove the seeds from the peppers and discard (feel free to leave a few, if you like a spicy chili).
You can do this in a hot pan, but I usually toast the chilies directly on a hot burner element, or over a hot burner flame. Over a medium-low temperature, toast the surface of the chilies by placing them in the flame, or directly on the burner surface. This will cause a quick blistering. Do not burn the chilies. Simply toast the surface for about 15 seconds, in a few spots around the chilies. This makes for a richer and more developed flavor.
Fill a bowl or measuring cup with about 4 cups of hot tap water. Place your toasted chilies in the hot tap water, so they may soften.
Pre-heat a large sauté pan over medium-high heat.
Turn your slow cooker to a low-temperature.
Season your beef with salt, pepper and cumin.
Add a little oil to the pan, and evenly distribute a handful of beef cubes. You want to sear the beef, to add a nice color, as well as lightly toasting the cumin. If you add too much beef, it will crowd the pan and your beef will not sear. When the beef has begun to color, toss the beef around in the pan, until all pieces of beef are very nicely caramelized and seared. Add the beef to the slower cooker. If you have more beef remaining, repeat this step until all the beef cubes are nicely caramelized and in the slow cooker.
Add your onions, garlic, fresh peppers and oregano to a blender, with a little salt and pepper.
Remove your chilies from the hot water and also add them to the blender. Save the water.
Puree the onion-chilies mixture. If you need to add any moisture to get the blender going, add small amounts of the water from the chilies, just to get the blender moving. You won't need much! Discard whatever you don't use.
Once the vegetables have been pureed, add the sauce to the slow cooker.
Stir the beef into the sauce.
Add the lid to the slow cooker and allow to cook on low for 8 hours.
Serve with favorite sides and grilled low carb tortillas!