Fit a fine mesh sieve or colander over a small bowl. Place a coffee filter in the sieve and add yogurt. Let drain for an hour.
Wash spinach, but don't dry. Heat a large skillet over medium heat. Add damp spinach and cook until wilted, stirring often. Drain and, when slightly cooled, squeeze to remove any remaining water.
Place skillet over medium-low heat and drizzle vegetable oil. Add onions and cook, stirring often, until golden. Stir in the garlic and cook just until fragrant, 30 seconds-1 minute. Add spinach and stir to combine. Remove from heat and let cool.
In a medium bowl, combine spinach mixture and drained yogurt. Season with salt and pepper. Refrigerate until ready to use.