Preheat the oven to 400 degrees Fahrenheit. In a large skillet over medium heat, warm the olive oil until shimmering. Add the onion, bell pepper, garlic, and a pinch of salt. Cook, stirring occasionally, until the onion and bell pepper are nice and tender, about 5 to 7 minutes. Meanwhile, squeeze as much moisture out of the thawed spinach as possible. Add the spinach and artichoke to the pan and cook for 1 minute, using your spatula to break up any hunks of spinach. Remove the skillet from the heat and add the sour cream, feta and two-thirds of the mozzarella (about 1 cup). Stir until the ingredients are all mixed together. Season to taste with additional salt, some black pepper, and a few dashes of hot sauce (more if you want more of a kick). Transfer the mixture to a small casserole dish (about 1-quart capacity) and spread it evenly into the pan. Sprinkle the remaining mozzarella over the contents. Bake until the top is golden brown, about 18 to 25 minutes. Let the dip cool for a few minutes (it's crazy hot) and serve with your dipping vehicle of choice. This dip keeps well for up to 4 days in the refrigerator, covered.