Classic latkes consist of grated potatoes, onions, and eggs, but it is kosher--and delicious--to include other vegetables, such as parsnips, sweet potatoes, or spinach, as long as you fry them in oil.
Melt butter in a large saute pan over medium heat. Add spinach, and gently saute until wilted, about 2 minutes. Transfer to a clean kitchen towel or paper towels, and wring out as much liquid as possible. Finely chop spinach, and transfer to a medium bowl.
Add remaining ingredients through pepper, and stir well to combine.
Fill a large, heavy-bottom skillet with about 1/2 inch oil. Place over medium heat until oil is almost smoking. (To test, drop a small bit of batter into the skillet; the oil should sizzle upon contact.)
Working in batches so as not to crowd skillet, carefully spoon about 2 tablespoons batter into oil for each pancake. Lightly tamp down to flatten. Cook until golden on each side, 2 to 3 minutes, turning once. Using a slotted spatula, transfer to drain on paper-towel-lined rack, and repeat with remaining batter. Serve immediately with applesauce and sour cream, if desired.