Prepare a grill for high heat. If using a charcoal grill, prepare a full chimney of charcoal. The charcoal is ready when a thin layer of white ash appears on each coal. Mound the coals directly under where the cast-iron pan will be placed.
Heat a large cast-iron pan on the hot grill (or on a stovetop over high heat). Season the scallops lightly with salt on both sides. Add the grapeseed oil to the pan.
To avoid sticking, the scallops should be added to the pan when it’s very hot. Shimmering streaks in the oil or light smoke rising from the pan are good indicators that it is hot enough. Add the scallops to the hot pan. Sear for 1 to 2 minutes, then flip and sear for an additional minute on the other side. Remove from the pan. At this point, the scallops should be evenly browned, but not fully cooked.
Melt the butter in the same pan, then add the shallot, stirring frequently until the shallot is translucent. Add the spinach and a pinch of salt. Sauté for about 1 minute.
Nestle the scallops back in and cook for an additional minute. Squeeze the juice from the lemon over the scallops and spinach to finish.