Prepare the crumb topping: Combine flour, sugar, and margarine. Mix to form coarse crumbs. Set aside.
Make the dough: Begin by using 4 cups of warm water. Combine water, yeast, and sugar in the bowl of an electric mixer; stir to combine. Stir and let sit 5-10 minutes.
Add eggs, oil and salt. Add 1/3 of the flour, mix until combined. Add another 1/3 flour, and mix just until combined. Add the last 1/3 of the flour. Add remaining water (or just as much as needed for a nice sticky dough). Mix for 10 minutes on high speed.
Remove dough from mixer and place in a large mixing bowl. Smear the top of the dough with a little oil. Cover and let rise 2 hours.
Prepare baking sheets: spray with baking spray or line with parchment paper or silicone mats.
Turn dough out onto a clean countertop. Divide dough into 6 equal parts. Braid each part and place in pans. Cover and let rise on the counter until it doubles in size, 1 to 1½ hours). (See notes.)
Preheat oven to 350ºF.
Brush challah with beaten egg and place crumbs on top. Bake in oven 45-50 minutes, tops should be golden and bottom should sound hollow when you tap it. (See notes.)