1.Place rack in upper third of oven and preheat oven to 425°F. Line baking sheet with parchment paper or stack two sheets together (to keep bottom of scones from over-browning).
2.In mixing bowl, combine flour, cornstarch, baking powder, baking soda and salt, whisking to blend. Stir in cheese, spinach and tomatoes.
3.In small bowl, whisk together yogurt, egg and oil until well combined. Stir into flour mixture until no dry ingredients remain.
4.Turn dough out onto floured cutting board. Sprinkle top with more flour and knead about six times. Press dough into 1-inch-thick circle (about 7 inches in diameter). Cut circle into 8 wedges. Transfer scones to prepared ungreased baking sheet.
5.Bake in upper third of oven for 10 to 14 minutes or until golden. Slide spatula under bottoms to release if necessary. Serve warm or at room temperature.
If your sundried tomatoes seem leathery, cover them with boiling water and let stand for a few minutes until softened, then drain and chop. If substituting oil-packed sundried tomatoes, blot excess oil with paper towels.