Heat the oil and butter in a large nonstick skillet over medium-low heat until shimmering. Add the red onions and cook, stirring occasionally, until softened and starting to brown, about 15 minutes. Add the green onions and garlic and cook for 2 minutes longer. Stir in the spinach; cook until the leaves begin to wilt. Add the chives, cilantro, parsley and dill, and sprinkle with salt and pepper. Take the skillet from the heat and stir in the pine nuts.
Beat the eggs, milk and some pepper in a blender on low speed for about 30 seconds. Pour the eggs over the veggie mixture. Transfer the skillet to the oven and bake until the omelette is just set and lightly golden brown on top, 20 to 25 minutes. Cool on a baking rack for 5 minutes, then place a large platter over the top of the skillet and flip the omelette onto the platter, bottom-side up. Slice into wedges and top with a dollop of yogurt if desired.