In a mixing bowl, mix the flour, baking powder and salt together, blending well. Next, add the egg and vinegar and then stir in the soda water. Keep cold until use and stir again before using.
Preheat a deep saucepot with the oil to 350 degrees F.
Dip the sardines in the cornstarch, then the tempura batter and then float in the fryer. The fish should be well-coated but not drenched. Cook 3 to 4 minutes, in batches if necessary, flipping over during the process to ensure even cooking. Remove from the fryer to paper towels, allowing the sardines to drain any excess oil. Hold warm until serving with the Tonatto Sauce.
In heavy-bottom saucepan over medium-high heat, heat the chicken stock, garlic, salt and pepper until simmering. Reduce the heat to medium and simmer for 10 to 15 minutes.
Next, in a saucepan over medium-high heat, melt the butter. Slowly add the flour with a whisk, stirring to incorporate. Continue to cook until the butter and flour begin to thicken, this takes about 3 minutes. Once thickened, remove from the heat.
After the stock has simmered and reduced by 25-percent original volume (or 1 cup), add the milk and cream and to stir allow the stock and dairy products flavor each other and create a soup base, about 10 minutes.
Next, add the roux to the soup base and stir in ensure smoothness. Finally, add the tuna, squash, zucchini and oregano over medium-low heat. Continue cooking for 20 minutes. Puree and adjust the seasoning.