Best Skinny Eggplant Rollatini with Spinach

medium Italian eggplants, cut lengthwise into 10 (1/4-inch thick) slices (21 oz total when sliced)

kosher salt and fresh black pepper,

quick marinara sauce

large egg

part skim ricotta cheese

grated Pecorino Romano cheese, plus more for serving

frozen spinach, heated through and squeezed well

garlic clove, minced

shredded part-skim mozzarella (Polly-O)