Preheat grill to medium-high (400°F to 450°F) or a grill pan to high. Brush chayote squash and onion with 1 Tbs. canola oil, then sprinkle with salt. Place on preheated grill. Cook, turning once, until charred and tender, 4 to 5 minutes per side for squash (for grill pan, 6 to 7) and 4 minutes per side for onion (for grill pan, 5 to 6). Remove from grill and coarsely chop. Place in a medium bowl and stir in spinach. Set aside.