For Soup:
For Grilled Cheese:
Heat butter in a large pot over medium heat. Add onion and cook, stirring, until softened, about 10 minutes.
Remove the pot from the heat. Transfer onions to a blender jar, add water and blend until smooth. Return the onion mixture to the same pot.
Add the baking soda, sugar, salt, black pepper and tomatoes. Place the pot over low heat and simmer for about 15 minutes. Whisk in milk. Do not boil as the milk may separate, which means it will still taste great but not look as good! Taste and hold warm until serving time.
Heat pan on medium heat or oven to 450°F (230°C).
Spread each slice of bread with butter. Place half of the slices buttered side down with a generous 2 Tbsp cheese or 2 slices of cheese. Top with other half of bread slices, buttered side up.
In a pan: Add grilled cheese and flip when one side is golden brown. Brown the second side. In an oven to bake all at once: Place grilled cheeses on an oven sheet pan for about 6-8 minutes, flip and bake for 6-8 minutes further until golden brown.
