Hi Mumukshu,
Welcome here!
I think you will find it easy to eat vegetarian on Fast days. You can eat what ever you like, so long as you stay under the maximum calorie count. But low GI and protein tend to keep people fuller for longer. Vegetables and dal!

I just have cuppas through my fast days, and then an enormous bowl of soup at the end of the day.

A very small portion (half size) of one of your normal meals (and you should be able to have two small meals on a fast day) would probably work. You need to count up the calories to check though.

Good luck.

mumukshu, this is a favorite at our house:

Mediterranean Vegetables with Brown Rice: 281 calories 7.1 g fat 10.4 g fiber 16.4 g protein 40 g carbs 332 mg Calcium PB GF
2 cups eggplant, peeled & cubed OR 1 and 1/2 cup bell peppers, cubed 2 cups tomato, cubed 2 cups zucchini, cubed 2 cloves garlic 1.5 tsp oregano ½ cup chickpeas, rised and drained per serving: 1 oz mozzerella, shredded 1/4 cup brown rice or couscous, cooked
Prepare all the veg., and put them in a saucepan. Simmer, covered, until cooked through. If watery, remove lid and continue to simmer. Add oregano, salt and pepper to taste, and the chickpeas, and keep warm. Prepare the brown rice or couscous and place it on one side of the plate. Spoon 2 cups of the vegetables next to the couscous. Top with cheese and tuck into it!

Red Beans & Rice: 295 calories 1.2 g. fat 13 g. protein 57.5 g. carb PB GF This is the old classic recipe for the ‘perfect protein’. Once we were afraid that we wouldn’t like it, since it sounds bland, but we do like it. Yeah, you’re right, the carbs are way out of control, but these complex carbs are really good. HINT: This is enough for 4 servings! Either have a gathering or cut the recipe or freeze for later.
1 and 3/4 cups brown rice, cooked ½ cup celery, chopped ½ cup sweet yellow or other color pepper, diced 1 cup onion, chopped 2 cloves garlic, minced ¾ cup crushed tomato oregano black pepper 1 and 3/4 cups red beans, drained and rinsed [Goya brand is very good] ½ cup green beans or peas
Cook the rice. Saute the celery, pepper, and onions for 5 minutes. Add the garlic and cook 5 minutes more. Add the tomato sauce and seasonings. Stir in the red beans and heat through. When the rice is done, add 1 and 3/4 cups to the mixture. HINT: if there is extra rice, it freezes well. Serve with the cooked green vegetable.