Cut eggplant into approximately 1" cubes and place on the prepared baking sheet.
Drizzle eggplant with 2 Tablespoons of the olive oil and sprinkle with salt, pepper & oregano.
Roast eggplant in the oven for 30 minutes, tossing halfway through, until browned.
While the eggplant is roasting, bring a large pot with 4 quarts of water to boil over medium high heat. Once water begins to boil add 1 Tablespoon of salt and then add in dried pasta. Stir and cook for 10-12 minutes or until al dente. (Cooking times may vary with gluten free pasta)
When pasta is done cooking, reserve 1/2 cup of the water and drain the rest from the pasta. Set aside.
When the eggplant is done roasting, heat the remaining 1 Tablespoon of olive oil in the pot that the pasta was cooked in over medium-high heat. Add in the roasted eggplant and baby spinach. Cook while stirring occasionally until the spinach just wilts, about 2-3 minutes.
Reduce the heat to medium-low and add in the marinara and reserved pasta water. Stir and allow to simmer for 1-2 minutes to heat the sauce through.
Add cooked drained pasta and toss with the sauce to combine everything.