I’m hearing you and I understand where you’re coming from. I personally wouldn’t do this though and here’s why:
Bread (white bread in particular) is high on the glycemic index, filled with loads of (processed) carbs, and spikes your blood sugar massively, which leads to a corresponding insulin spike. Higher insulin levels tend to lead to our body prioritizing glucose as the main fuel source, while simultaneously prioritizes storing fat. This generally means the higher and longer you spike your insulin the less fat you’d be burning. Quintessential to fasting is keeping your insulin levels low enough to get to those fat stores and burn them off as the primary fuel source over blood glucose.
I don’t think bread, most of which is very processed and filled with sugar, along with also heavily processed butter and/or jam, are the best option if your goal is fat lose. Now with all that said, will you still burn fat and lose weight eating only 2 slices of bread (with condiments on them)? Yes. If your caloric intake is only 500-700 calories (or how ever much the meal is) and you’re in a roughly 1300-1500 caloric deficit (I’m estimating your BMR is about 2000 calories), your body will burn fat. You’d essentially be doing a low calorie OMAD (one meal a day). Fasting for that long (around 23 hours in a day) then eating only 500-700 calories in a day is going to lead to fat and weight lose anyway you splice it. It just wouldn’t be as fast or efficient as if you ate a meal higher in fat and protein, or even more complex carbs, and lower in processed carbs. Bread and condiments are also calorically dense, meaning you don’t get as much of your fill eating 2 slices of bread as opposed to eggs, chicken, and leafy greens. It doesn’t have as strong of a satiety-inducing response. It generally does the opposite really, leading to stronger hunger cravings.
My advice? Figure out how many calories you’d actually be consuming with your bread and condiments of choice, then choose a more satiating meal (think more greens, protein, complex carbs). Like I said, you’re essentially going to be doing OMAD, so you might as well make the best of that one meal.
Trying to figure out what is sustainable and works is not easy, but it’ll generally work out in the end if you stay committed to improving yourself. Best of luck.
Besides the obvious of losing out on the insulin resistance and autophagy, you’ll also be starving 24/7. A high carb diet means your body is begging for glucose when you’re trying to fast, and if you don’t go longer than a day without it, you’re going to be hungry and miserable all the time.
What your describing is OMAD, like the other comment said, but you’d be doing a horrible version of it. If you’re going to do it, you’re better off doing something that keeps you in ketosis. An omlette with a few eggs, some veg, and some cheese will be similar in calories, fill you much better, and keep you in fat burning mode.