Preheat the oven to 400 degrees. Line a large baking pan with parchment. Set aside.
In a large skillet or pot, heat oil over medium-high heat, then add the vegetarian sausage, breaking it up with a wooden spoon to crumble in the pan. Add onion and cook for about 5-6 minutes or until the onions are slightly softened.
Add the celery, garlic and about a teaspoon of salt. Stir well; reduce heat to medium. Continue to cook the mixture, stirring occasionally, until they have softened about 7-8 minutes.
Add the sage and the stock, scraping up any brown bits from the bottom of the pan, and increase the heat once again to bring the mixture to a boil. Once it begins to bubble, turn off the heat and stir in the spinach, parsley and cubed bread. {This would be a good time to sneak a taste and make sure it is well-seasoned.}
Add the beaten eggs and mix until fully combined, then spread the mixture onto the prepared baking pan in an even layer. Distribute the cubed butter pieces over the top.
Bake for 20-25 minutes or until the top turns a deep golden brown and the edges are crisp. Serve while hot.