Note: Escarole (a member of the chicory family) is a leafy green with a sweet-bitter taste. It's tender when cooked and can be used in braises, soups or stews. Look for it near other greens in the produce section, but do not confuse it with curly endive. Escarole has large, relatively broad leaves resembling densely packed leaf lettuce (curly endive has tight and finely ruffled leaves). Tip: To oil a grill rack: Oil a folded paper towel, hold it with tongs and rub it over the rack. (Do not use cooking spray on a hot grill.)
Place 1 tablespoon oil and garlic in a large saucepan over medium heat. Cook, stirring, until the garlic is sizzling and fragrant, 1 to 2 minutes. Add crushed red pepper; cook, stirring, until fragrant, about 30 seconds. Add tomatoes and oregano; bring to a boil. Reduce to a simmer and cook until juicy, about 3 minutes. Stir in shrimp and escarole; cook, stirring, until the escarole is wilted, about 2 minutes. Remove from the heat, cover and keep warm.
Oil the grill rack (see Tip). Grill polenta slices until hot and slightly charred, 3 to 4 minutes per side.
Divide the sauce among 4 shallow bowls or plates. Top with the polenta slices, sprinkle with olives (if using) and drizzle each serving with 1/2 teaspoon oil. Serve immediately.