Place a large skillet over medium heat. Add oil and brown sausage, breaking it up as it cooks, about 3 to 4 minutes. Transfer sausage to bowl and set aside.
Add remaining oil to skillet and sauté onion and garlic for 2 to 3 minutes. Season with salt and pepper.
Add spinach and continue to sauté until spinach wilts, 2 minutes.
Pour onion mixture into bowl with sausage and set aside.
Grease 9"x13" baking dish with 2 tablespoons butter.
In a liquid measuring cup whisk together eggs and cream until completely combined. Generously season with salt and pepper.
Add half bread pieces into the bottom of the baking dish in an even layer. Top with half the sausage, spinach and onion mixture. Pour half the egg and cream mixture over strata and sprinkle with half shredded cheese.
Repeat previous step until everything has been used and baking dish is full. Top dish with half green onions.
Tightly cover strata with foil and refrigerate overnight (or at least one hour).
Preheat oven to 375˚F. Bake covered strata in oven for 30 minutes. Uncover and continue to bake for an additional 15 to 20 minutes or until the top has browned and slightly crisped at the edges and the center has just set.
Cool strata for 10 to 15 minutes, top with remaining green onion and serve.