Espagnol Sauce can be made by adding brown roux in brown stock. So first make brown stock.

Take beef bones saw 2 inch long and brush with tomato paste properly. Now roast it till caramelization in preheated oven.

Remove bones in a pot. Now put chopped mirepoix in the roast container and mix. Roast it for 2 mins and take out and mix with roasted bones.

Add water double the quantity of bones and add leek, garlic, bayleaves, parsley then let it boil for 4 hours, skim the impurities or foams out.
Strain the water in a pot. Brown Stock is ready.

Now heat the butter in a pan and add flour. Use whisk to mix the butter and flour. Heat the roux till it turn brown. Brown roux is ready and keep cool.

Heat stock in a pot and add the roux in as per required to make perfect thickness. One of it should be hot or cold.

Maintain the thickness and Espagnole sauce is ready. Further adding equal amount of brown stock in brown sauce and heat to reduce by half, the sauce is Demi-glace.