Cut the eggplants into quarters lengthwise and season with salt. Heat 2Tbsp olive oil in a large pan over medium-high heat. Sear the cut sides of the eggplant until golden, ca. 2min. per side. Add more oil as needed when flipping the eggplant. Tip: place a second pan on top of the eggplant as it sears to weigh down the pieces and get an even sear. When done, remove the slices and set aside.
Peel, halve and thinly slice the onions. Reserve ¼ of the onion slices for the salad. Peel and mince or press the garlic with a garlic press. In the same pan, melt 2Tbsp butter on medium heat. Add the onion and garlic and sauté for 3–4min. to soften. Then stir in the the garam masala and ¾ of the tomato paste. Sauté for ca. 1min. until fragrant and toasted.
Gradually pour in the coconut milk and 80ml of water, stirring to combine. Season to taste with salt and pepper. Bring to a gentle simmer, then nestle the eggplant pieces into the sauce. Cover and gently simmer for ca. 8min., until the eggplant is tender when pierced with a fork.
Meanwhile, in a small pot, bring enough water to a boil for the eggs. Add the eggs carefully to the boiling water and boil on medium heat for ca. 6min. for soft eggs. Tip: If you want your eggs hard-boiled, cook for 2 more minutes. Drain the eggs, rinse with cold water and peel.
Push the eggplant to the sides of the pan, and stir ½ the spinach into the sauce in the center of the pan, letting it wilt. Add 1–2Tbsp water, if needed. Spread the eggplant and spinach evenly across the pan. Break the feta into chunks and scatter them over the top of the pan. Cover and gently simmer for ca. 5min. to soften the feta. Let rest uncovered when done.
In a medium bowl, mix 1Tbsp olive oil, 1Tbsp vinegar, the reserved onion and a large pinch of sugar, salt and pepper. Toss the remaining spinach in the dressing. Roughly chop the fresh coriander and stems. Sprinkle it over the curry. Halve the eggs. Serve the curry in bowls, topped with egg, and the spinach salad alongside.