Preheat the oven to 450°F.
Slice onion and squash, then mince the garlic.
In an oven-safe skillet, heat oil and add onion. Sauté onion until tender.
Add in garlic and stir for 30 more seconds, and then add milk. Cook until milk starts to bubble slightly and then add parmesan.
Add sliced summer squash to the mixture and cook for 4-5 minutes.
Remove from heat and top with gouda. Place skillet in the oven to finish cooking for about 15-20 minutes or until gouda has started to golden on top. Serve hot!