Heat olive oil in large skillet over medium-low heat. Add garlic and cook 1-2 minutes to infuse oil with garlic. Using slotted spoon, remove garlic and discard.
Add spinach to olive oil and cook 1-2 minutes, until wilted. Add nutmeg and season to taste with salt and pepper. When spinach is wilted, stir in sour cream and artichoke hearts.
Butter 1 side of each bread slice. Place 4 slices, butter-side-down, on griddle over medium heat. Place 1/8 cup monterey jack on each slice. Divide spinach mixture evenly between 4 slices of bread, then top with additional 1/8 cup of cheese. Top with remaining 4 slices of bread, butter-side-up.
Grill until golden brown on each side, about 3-5 minutes per side. Let rest for 2 minutes before serving.