In the bottom of a large slow cooker, mix together the chicken broth (1/2 cup), melted butter (2 tablespoons), and half of each of the dried oregano (1/2 tablespoon), kosher salt (1/2 teaspoon), and black pepper (1/2 teaspoon). Squeeze the juice out of the quarters of the lemon into the mix as well.
Add the chicken breasts to the slow cooker and, using tongs, coat each side of the chicken with the mixture. Nestle the juiced lemon quarters around the chicken breasts. Season the top of the chicken breasts evenly with the remaining 1/2 tablespoon dried oregano, 1/2 teaspoon kosher salt, and 1/2 teaspoon black pepper.
Cover and cook on high for 1.5-2 hours. Check for doneness by making sure the internal temperature on the thickest part of the largest breast is 165 degrees F.
Remove the chicken breasts from the slow cooker. Allow to cool for 10-20 minutes, then place in an airtight container to store whole. To slice, remove the chicken breasts to a plate or cutting board and tent with foil and allow to rest for 10 minutes, then slice against the grain. If shredding, use two forks to shred the chicken breasts (directly in the slow cooker if tender enough, if not, remove to a cutting board and place back in the liquid in the slow cooker when done), and serve or store (with liquid) in an airtight container in the fridge or freezer.
Ingredient Substitutions: Water can be used instead of chicken broth in a pinch- you may need to add more seasoning to boost the flavor. Other seasoning, like Italian seasoning, thyme, or Greek seasoning, can be used instead of the oregano. And the butter can be substituted with olive oil, or omitted for a lower fat version.
For a very simple version, the oregano and lemon can be omitted. Just season with salt and pepper.
Variations: Try using taco seasoning instead of oregano and lime instead of lemon for a southwestern flavor. Or add a bunch of fresh herbs in the mixture- anything you have growing in your garden! You can also add sliced fresh onion and/or minced garlic, and some veggies like potatoes, carrots, green beans, etc., under the chicken breasts for a complete meal.
I don't recommend cooking these on low for longer. The texture of chicken breasts can be stringy and mushy if cooked for too long in the slow cooker. That said, if you really want to, you can cook for 3-3.5 hours on low.