In an 8-quart pot, bring 4 quarts of water to a boil with 1 Tbs. salt. Carefully put the onions in the boiling water. Cook until they’re tender but still retain their shape, 25 to 30 minutes. Drain and let cool.
Position a rack in the center of the oven and heat the oven to 375°F.
When the onions are cool enough to handle, cut them in half crosswise and trim the root ends. Holding an onion wrapped in a clean towel, use your fingers to carefully remove the core from each half of the onion, leaving a 1/2-inch-thick shell. Cut a small amount off the bottom of each half, so they sit upright. Repeat with the remaining onions. Roughly chop and reserve 1 cup of the removed onion core.
In a 12-inch skillet, heat the oil over medium-high heat until shimmering. Add the garlic and ginger and cook, stirring, until lightly browned, about 1-1/2 minutes. Add the garam masala and cumin and continue cooking, stirring, until fragrant, about 30 seconds more. Reduce the heat to medium, add the raisins and 2 Tbs. water, and cook, stirring, until the raisins are plump, about 1 minute. Add the reserved chopped onions and the flank steak and cook, stirring frequently, until just heated through, 1 to 2 minutes. Add the spinach and cook, stirring, until wilted, 1 to 2 minutes. Off the heat, stir in the crumbled feta and lemon juice and season to taste with salt and pepper.
Spoon the filling into the onion halves, mounding it slightly. Put the onions on a rimmed baking sheet and bake until the tops just begin to brown, about 15 minutes. Serve.
You can cook and refrigerate the onions up to 3 days ahead.