Dice the yellow onion into 1/2-inch pieces, cut the red bell peppers into 1-inch squares, and mince the garlic. Pat the chicken breasts dry with paper towels, then season both sides generously with kosher salt, black pepper, and Italian seasoning. I like to let the seasoned chicken sit for a few minutes—this helps the flavors penetrate the meat rather than just sitting on the surface.
Heat the extra-virgin olive oil in a large skillet over medium-high heat until shimmering. Sear the seasoned chicken breasts for 1-2 minutes per side until they develop a light golden crust. You're not cooking them through—just building flavor and color. This browning step adds depth that slow cooking alone cannot achieve, even though the chicken will finish cooking in the slow cooker.
In a small bowl, whisk together the lemon juice, minced garlic, chicken broth, and a pinch of additional salt. This creates your sauce base that will infuse the chicken with bright Mediterranean flavors. I find mixing these wet ingredients separately first ensures the garlic distributes evenly rather than clumping in the slow cooker.
Add the diced onions to the bottom of your slow cooker, then arrange the seared chicken breasts on top. Scatter the kalamata olives, red bell peppers, and drained capers evenly around and over the chicken. Pour the lemon-garlic broth mixture from Step 3 over everything, making sure the liquid reaches around the chicken for even cooking.
Cover the slow cooker and cook on low for 4 hours (or on high for 2 hours if time is limited). The chicken is done when it reaches an internal temperature of 165°F and shreds easily with a fork. The low, gentle heat allows the Mediterranean flavors to meld beautifully while keeping the chicken tender and juicy.
Taste the finished dish and adjust seasoning with additional salt and pepper as needed. Ladle the chicken and vegetables into serving bowls along with plenty of the flavorful broth. Garnish generously with fresh thyme leaves just before serving for a bright, herbaceous finish that contrasts with the rich, briny flavors of the olives and capers.