Pat pork chops dry, then season all over with salt and pepper.
Heat olive oil in a skillet over medium-high. Sear chops 2–3 minutes per side until lightly browned.
Spread sliced onion in the slow cooker. Nestle seared chops on top.
Whisk maple syrup, Dijon, broth, vinegar, soy sauce, garlic, thyme, and rosemary. Pour over the chops.
Cover and cook on LOW 5–6 hours or HIGH 2.5–3 hours, until tender but not falling apart.
Transfer chops to a plate. Turn cooker to HIGH. Stir cornstarch with water, then whisk slurry into the sauce.
Cook 5–10 minutes until the sauce thickens and turns glossy. Stir in butter, then taste and adjust seasoning.
Return chops to warm briefly or plate and spoon sauce over top to serve.