Mediterranean eggplant ratatouille

I made the recipe you sent me from The Meat Free Monday Cookbook but didn't have some of the ingredients so it changed considerably into this; very delicious! I'll definitely be making it again soon.

3 Tbsp olive oil
1 medium onion, chopped
1 stick celery, chopped (or 3 small)
2 fat garlic cloves, crushed
1 eggplant, cubed
1 - 1.5 cup Trader Joe's Harvest Grains blend (a mixture of Israeli/pearl couscous, red and green orzo, split dried garbanzo beans, and red quinoa). Substitute with whatever grains you can find, or potatoes.
1 tsp dried oregano
1 large (28 oz/800g) can/2 medium (14.5oz/400g) cans chopped tomatoes
2 cups/400g vegetable stock (or a bouillon cube and boiled water)
10-12 mushrooms, chopped
1 large apple, chopped (or 3 small)
1-2 Tbsp capers
2 oz/125g feta, crumbled

Heat some of the oil in a large soup pot, add the chopped onion and celery and sprinkle with a little salt. Cover with a lid (this helps them sweat but not brown). While the onion is cooking, chop the eggplant. Add the remaining oil and the eggplant and cook 4-5 more minutes, until the eggplant begins to soften. Add the tomatoes, stock/water, grains, oregano and bring to a boil. Reduce heat, simmer for  25-30 mins until all the vegetables are tender and the grains are cooked (check halfway through to stir and add more water if necessary). At the end, it should be thick & chewy, not soupy.

Add the capers and heat through, and salt & pepper to taste. Serve with the feta.

Optional: additional vegetables like zucchini and red bell pepper, olives, and sprinkle with parsley and pine nuts (or more affordable: sunflower seeds). You can definitely make this ahead and let it sit, it will improve.