Cut 1 TBSP of butter off the stick of 1/3 cup and heat it in a frying pan on medium.
Add in the diced onions. Let them cook for a minute or two, then add the zucchini, added a few dashes of salt and pepper. Saute vegetables for about 5 minutes, then turn off the heat.
While the zucchini is sautéing, begin making a roux (fancy word for white sauce) for your chowder. Open the cans of chicken broth and have them close at hand.
Melt the remaining amount of butter in a medium-sized saucepan over medium heat. Add in sage, garlic powder and parsley.
When the butter begins to sizzle, add a small amount of the chicken broth (maybe 1/4 cup). Let mixture heat back up, then begin adding the flour a little at a time, whisking until smooth.
In alternating amounts, add more chicken broth, then flour, until all the flour is added.
Add in lemon juice, tomatoes, corn and remaining chicken broth. Increase the heat slightly (I had my stove on "6") stirring occasionally.
Once mixture looks to be nearly boiling, decrease heat back to medium and add in the cheese. Whisk gently until melted.
Whisk gently until melted. Add in evaporated milk and the zucchini/onion mixture. Simmer soup for about 10-15 minutes.
Serve with additional cheese on top if desired.