In a large bowl, combine the cold mashed potatoes with olive oil and mix well.

Add salt, black pepper, Italian seasoning, garlic powder, and onion salt to the potato mixture. Stir until seasonings are evenly distributed.

Gradually incorporate 1 cup of flour, adding more as needed until you achieve a soft, pliable dough that's no longer sticky but still light and somewhat delicate.

Turn the dough onto a well-floured surface and knead about 20 times to develop just enough gluten to hold the gnocchi together.

Divide the dough into four equal portions and roll each into a rope approximately 12 inches long.

Using a sharp knife or bench scraper, cut each rope into half-inch pieces.

Create the classic gnocchi ridges by gently rolling each piece across the tines of a fork.

Bring a large pot of generously salted water to a rolling boil.

Cook the gnocchi in small batches to prevent sticking, dropping them carefully into the boiling water.

When the gnocchi float to the surface (usually within 2-3 minutes), they're done. Remove with a slotted spoon and transfer directly to your sauce of choice.