Preheat the oven to 350 degrees(F).
In a large non-stick skillet, heat the olive oil on medium high.
Add the onions, garlic and potatoes. Season with a pinch of salt and pepper. Sauté, reducing heat as needed until the potatoes are starting to get tender and the onions are translucent. About 7 to 9 minutes. Remove from heat.
In a large mixing bowl, whisk together the vegan egg, vegan cream or non-dairy milk, salt and pepper.
Scoop the sauteed onions, garlic and potatoes into the bowl with the vegan egg.
Then add in the sundried tomatoes, olives and spinach. Stir everything together for a minute or until fully combined.
Pour the vegan egg(omelet) mixture into a 9 x 13 inch sheet pan that has been lined with parchment paper or a silicone mat(you can use a larger one, it just won't fill it completely, but it will work out fine!)
Spread the omelet out evenly into the sheet, sprinkle the vegan feta or goat cheese on top of the omelet if using. Then bake for 20 to 25 minutes or until the omelet is firm.
While the omelet bakes, you can cook the vegan sausage patties however you want, depending on what kind you have.
Once the sheet pan omelet is done, let it cool for about 5 minutes, then cut into 12 equal squares.
Build your breakfast sandwiches, take an English muffin, slice it open, put an omelet square on the bottom, then a sausage patty, then a slice of vegan cheese. Put the top on the sandwich.
At this point, you can eat one or as many as you want. Or, you can freeze them. Wrap each sandwich individually in parchment paper and then store them all together in a large gallon sized Ziploc bag or 2. Put them in the freezer until ready to heat and eat.
When ready to microwave and eat, remove a sandwich from the bag and the parchment paper. Wrap in a paper towel and microwave for 1 minute, flip and microwave for 1 more minute. Let cool for a few minutes and then eat!