Preheat oven to 375° F. Spray 9” x 13” baking dish with nonstick cooking spray.

Heat a 12 inch skillet on medium heat and add 1 Tbsp olive oil. Add spinach and ¼ tsp salt to skillet and cover for 3 minutes to wilt. Uncover and stir to make sure all the spinach is evenly wilted.

Transfer cooked spinach to a plate lined with paper towels to drain and cool, making sure to leave excess liquid behind in the skillet.

Discard spinach liquid and wipe out skillet. Return skillet to medium heat and add 3 Tbsp olive oil and 3 Tbsp butter. Once butter has melted stir in diced onions, black pepper, nutmeg, dry mustard and dry crushed tarragon. Sauté for 5 minutes until onions are translucent but not brown.

Stir in all purpose flour until evenly incorporated. Cook for about 30 seconds.

Slowly whisk in cold milk about ⅓ of a cup at a time. It will be really thick at first but will smooth and thin out as you add more milk.

Transfer sauce to a large mixing bowl and stir in 1 tsp salt, cheddar and parmesan cheeses. Taste for seasoning and add more salt or pepper if desired. Cover and let cool until temperature is around 90° F.

Add ⅓ cup of cooled cheese sauce to egg yolks and whisk to temper. Add tempered egg yolks back to the rest of the cheese mixture and stir to incorporate.

Squeeze spinach between additional paper towels or a very clean dish towel to remove excess liquid. Transfer to a cutting board and roughly chop. Add cooled, chopped spinach to cheese sauce and stir thoroughly making sure all the spinach is broken up and evenly distributed.

Add cream of tartar to egg whites and whip on medium speed until stiff peaks form. (When you lift the whisk out the egg whites, a peak will form on the whisk that will stand straight up. Properly whipped egg whites won’t slide around the bowl if you tip it on its side. This will take around 5 minutes depending on your mixer. You may want to finish whisking by hand as you get close to stiff peaks to avoid over mixing and “breaking”the egg whites.)

Add ¼ of the whipped egg whites to the cheese mixture and fold in. Add another ¼ of egg whites and fold in thoroughly. Add remaining egg whites and fold in gently until fully incorporated, but not deflated.

Pour batter into prepared baking dish and place on a baking sheet. Transfer to the center of your preheated oven and bake for 40 minutes until the soufflé is a deep mahogany brown, puffed and registering 170° F in the center when a thermometer is inserted from the side. Temperature will continue to rise to 180° F after you take it out of the oven.

Let rest 5 minutes, slice and serve immediately!