Preheat oven to 350°F (180°C).
Cut chicken breast into ½ inch (1.25 cm) strips lengthwise and then cut again crosswise into ½ inch (2.5 cm) square pieces.
Heat 1 tsp (5 mL) of olive oil in a medium size non-stick pan over medium heat. Sauté chicken breast with red onion until starting to brown and is thoroughly cooked, approximately 10 minutes. Remove from pan. Add the rest of the olive oil to pan and heat on low.
Beat eggs with yogurt and horseradish (if using), Dijon mustard and salt until well combined. Add more horseradish if you like a zippier omelette. Pour into the heated, ovenproof skillet or baking dish.
Top egg mixture with the spinach and grated carrot followed by the sautéed chicken and onion. Press spinach down into filling if it is sticking up as it will dry out during baking.
Bake in a hot oven on the centre rack for 30 minutes or until the omelette is set and just lightly browned. The omelette is done when a knife inserted into centre comes out clean. Remove from oven.
Top with the grated Asiago cheese. Let sit 5 minutes before serving. To serve cut into 6 wedges and serve with salad.