Instead of using the soup cubes, you can boil beef bones. That is how my mom makes it. It takes some times, but the flavor is great.
http://www.vietworldkitchen.com/bookshelf/articles/pho_SJM.htm
Something like this I guess:
Making Pho at Home
By Andrea Q. Nguyen
Special to the Mercury News
It's convenient and fun to eat pho out, but nothing beats a homemade bowl.
What makes the homemade version dac biet (special) is the love and care put into the broth -- the cornerstone of pho. Multidimensional in fragrance and flavor, homemade broth will beat out those prepared in restaurants any day.
I've learned to make pho from listening to my mom and other women, reading cookbooks in Vietnamese and English, and emptying many bowls. Here are some insights:
1. Start with good beef bones: Avoid neck bones. Look for knuckle bones and leg bones that contain marrow. At Asian markets, you'll find beef bones cut and bagged in the refrigerated section. Vietnamese markets will sometimes have the leg bones at the butcher counter. You can specify how you want them sawed; ask for two- to three-inch sections.
From eating pho in Vietnam and observing how the cows there live low-key lives grazing in the countryside, I was inspired to make pho broth from the fragrant bones of grass-fed and natural beef. The experiments have consistently yielded amazing results, with the essence of beef captured every time. To find the bones, ask a butcher who breaks down large beef carcass sections into small retail cuts. [Also check these sites for sources for natural, organic or grass-fed beef: Eatwellguide.org, Localharvest.org, Eatwild.com]
2. Aim for a clear broth: This is achieved by parboiling and rinsing the bones, which greatly reduces the amount of residue in the broth. You may think you're pouring essential flavors down the drain, but you're not. The bones exude their essence during the three-hour gentle simmer. Cooking at a low heat also helps produce clear broth.
3. Char the onion and ginger: It imparts a wonderful brown color and deepens the overall flavors.
4. Leave some fat: Despite all the talk about obesity in the United States, I like some shiny globules of fat floating in the broth. They lend a richness that underscores pho's beefiness.
5. Serve it hot: To cook the raw beef and warm the cooked beef and noodles, the broth must be boiling when it's ladled into the bowl. But hot pho shouldn't be left to sit in the bowl. The noodles will absorb too much broth.
Posted Wed, June 9, 2004, copyright San Jose Mercury News