Position rack in upper third of oven; preheat broiler.

Heat oil in a large skillet over medium-high heat. Add onion and cook, stirring, until beginning to soften, about 2 minutes. Reduce heat to medium, stir in beef, flour and tomato paste, and cook, stirring, until the beef is mostly browned, about 3 minutes. Add broth, scraping up any browned bits with a wooden spoon. Bring to a boil and cook, stirring occasionally, until the broth is the consistency of thick gravy, about 4 minutes. Stir in spinach, 1/4 teaspoon salt and 1/4 teaspoon garlic powder; cook until the spinach is just wilted, about 1 minute. Remove from the heat.

Place squash in a fine-mesh sieve and gently press on it to extract excess liquid. Transfer to a bowl. Stir in the remaining 1/2 teaspoon salt and 1/4 teaspoon garlic powder. Divide the meat mixture among four 10-ounce broiler-safe ramekins. Top each with about 1/2 cup of the squash. Place the ramekins on a baking sheet.

Broil until heated through and bubbling around the edges, about 10 minutes. Sprinkle with cheese and broil until it is just melted, about 3 minutes more.

Equipment: Four 10-ounce broiler-safe ramekins