Trim loose skin and thick pockets of excess fat from the chicken thighs, then season both sides with kosher salt and black pepper.
Heat a large skillet over medium-high heat, then add the olive oil once the pan is hot.
Place the chicken thighs skin-side down in the skillet and sear for about 5 minutes, until the skin is deeply golden and releases easily from the pan.
Flip the thighs and cook for 2 minutes on the second side, then transfer them to a clean plate.
Add the red onion, carrots, potatoes, corn, and garlic to a 6-quart slow cooker and spread them into an even layer.
Arrange the seared chicken thighs skin-side up over the vegetables, then tuck the thyme and rosemary sprigs around the chicken.
In a medium bowl, whisk together 1 teaspoon kosher salt, 1/2 teaspoon black pepper, the chicken stock, flour, and lemon juice until smooth and free of flour lumps.
Pour the sauce around the chicken and over the vegetables, avoiding the browned skin as much as possible.
Cover and cook for 2 to 4 hours on High or 4 to 6 hours on Low, until the chicken reaches 165°F at the thickest point and the potatoes are tender.
Transfer the cooked chicken and vegetables to serving plates and remove the thyme and rosemary stems.
For a smoother sauce, strain the cooking liquid and season it with additional salt and black pepper to taste.
For crispier skin, place the chicken thighs skin-side up on a sheet pan and broil for 2 to 4 minutes, watching closely to prevent scorching.
Serve the chicken with the vegetables, warm sauce, and lemon wedges.