Heat one teaspoon of olive oil in a large skillet over medium heat on the stovetop.

Sauté the diced onion, crushed garlic, and sliced cremini mushrooms for 3-5 minutes, until lightly browned, to bring out their natural sweetness and umami. (If skipping the sautéing step: Add the diced onions, garlic, and sliced mushrooms directly to the slow cooker with broth. The flavor will still be delicious.)

Transfer the sautéed onion, garlic, and mushrooms to the slow cooker and pour the chicken broth over them.

Remove the skin from the turkey breast. Then, pat the turkey dry.

In a small bowl, thoroughly mix the salt, pepper, garlic powder, onion powder, thyme, rosemary, and sage.

Rub the seasoning mixture onto the turkey breast, evenly coating the top and bottom.

Heat the remaining teaspoon of olive oil in the skillet over medium-high heat. Sear the turkey breast 2-3 minutes per side, until golden brown. This adds depth of flavor.

Place the seared turkey, breast side up, on top of the sautéed onions, garlic, mushrooms, and broth in the slow cooker. (If skipping the sear: Place the seasoned turkey breast directly on top of the onions, garlic, mushrooms, and broth.)

Place the lid on the slow cooker and cook on low for 6-7 hours or on high for 3-4 hours, or until the internal temperature reaches 165°F.

Once fully cooked, transfer the turkey to a cutting board and cover it loosely with foil. Let it rest for 10-15 minutes to allow the juices to redistribute.

Leave the mushrooms and liquid in the slow cooker. Make the cornstarch slurry by mixing the cornstarch and cold water in a bowl.

Add the cornstarch slurry to the slow cooker. Turn the slow cooker to high and cook for 5-10 minutes, stirring occasionally, until the gravy thickens. Optional: Blend with an immersion blender for a smooth gravy, or leave it as is.

When ready to serve, pour the mushroom gravy over the turkey breast and top with optional fresh chopped parsley.